Copying over the rationale for publication here, for convenience: 

Rationale for Public Release

Releasing this report inevitably draws attention to a potentially destructive scientific development. We do not believe that drawing attention to threats is always the best approach for mitigating them. However, in this instance we believe that public disclosure and open scientific discussion are necessary to mitigate the risks from mirror bacteria. We have two primary reasons to believe disclosure is necessary:

1. To prevent accidents and well-intentioned development

If no serious concerns are raised, the default course of well-intentioned scientific and technological development would likely result in the eventual creation of mirror bacteria. Creating mirror life has been a long-term aspiration of many academic investigators, and efforts toward this have been supported by multiple scientific funders.1 While creating mirror bacteria is not yet possible or imminent, advances in enabling technologies are expected to make it achievable within the coming decades. It does not appear possible to develop these technologies safely (or deliberately choose to forgo them) without widespread awareness of the risks, as well as deliberate planning to mitigate them. This concern is compounded by the possibility that mirror bacteria could accidentally cause irreversible harm even without intentional misuse. Without awareness of the threat, some of the most dangerous modifications would likely be made for well-intentioned reasons, such as endowing mirror bacteria with the ability to metabolize ᴅ-glucose to allow growth in standard media.

2. To build guardrails that could reliably prevent misuse

There are currently substantial technical barriers to creating mirror bacteria. Success within a decade would require efforts akin to those of the Human Genome Project or other major scientific endeavors: a substantial number of skilled scientists collaborating for many years, with a large budget and unimpeded access to specialized goods and services. Without these resources, entities reckless enough to disregard the risks or intent upon misuse would have difficulty creating mirror bacteria on their own. Disclosure therefore greatly reduces the probability that well-intentioned funders and scientists would unwittingly aid such an effort while providing very little actionable information to those who may seek to cause harm in the near term.

Crucially, maintaining this high technical barrier in the longer term also appears achievable with a sustained effort. If well-intentioned scientists avoid developing certain critical components, such as methods relevant to assembling a mirror genome or key components of the mirror proteome, these challenges would continue to present significant barriers to malicious or reckless actors. Closely monitoring critical materials and reagents such as mirror nucleic acids would create additional obstacles. These protective measures could likely be implemented without impeding the vast majority of beneficial research, although decisions about regulatory boundaries would require broad discussion amongst the scientific community and other stakeholders, including policymakers and the public. Since ongoing advances will naturally erode technical barriers, disclosure is necessary in order to begin discussions while those barriers remain formidable.

There is also this essay from Jason Crawford and this piece from Asimov Press that are less technical description of the Science article

Thanks for sharing this, Aaron!

I agree the "Rationale for Public Release" section is interesting; I've copied it here:

Releasing this report inevitably draws attention to a potentially destructive scientific development. We do not believe that drawing attention to threats is always the best approach for mitigating them. However, in this instance we believe that public disclosure and open scientific discussion are necessary to mitigate the risks from mirror bacteria. We have two primary reasons to believe disclosure is necessary:

1. To prevent accidents and well-intentioned development If no serious concerns are raised, the default course of well-intentioned scientific and technological development would likely result in the eventual creation of mirror bacteria. Creating mirror life has been a long-term aspiration of many academic investigators, and efforts toward this have been supported by multiple scientific funders.1 While creating mirror bacteria is not yet possible or imminent, advances in enabling technologies are expected to make it achievable within the coming decades. It does not appear possible to develop these technologies safely (or deliberately choose to forgo them) without widespread awareness of the risks, as well as deliberate planning to mitigate them. This concern is compounded by the possibility that mirror bacteria could accidentally cause irreversible harm even without intentional misuse. Without awareness of the threat, some of the most dangerous modifications would likely be made for well-intentioned reasons, such as endowing mirror bacteria with the ability to metabolize ᴅ-glucose to allow growth in standard media.

2. To build guardrails that could reliably prevent misuse There are currently substantial technical barriers to creating mirror bacteria. Success within a decade would require efforts akin to those of the Human Genome Project or other major scientific endeavors: a substantial number of skilled scientists collaborating for many years, with a large budget and unimpeded access to specialized goods and services. Without these resources, entities reckless enough to disregard the risks or intent upon misuse would have difficulty creating mirror bacteria on their own. Disclosure therefore greatly reduces the probability that well-intentioned funders and scientists would unwittingly aid such an effort while providing very little actionable information to those who may seek to cause harm in the near term. Crucially, maintaining this high technical barrier in the longer term also appears achievable with a sustained effort. If well-intentioned scientists avoid developing certain critical components, such as methods relevant to assembling a mirror genome or key components of the mirror proteome, these challenges would continue to present significant barriers to malicious or reckless actors. Closely monitoring critical materials and reagents such as mirror nucleic acids would create additional obstacles. These protective measures could likely be implemented without impeding the vast majority of beneficial research, although decisions about regulatory boundaries would require broad discussion amongst the scientific community and other stakeholders, including policymakers and the public. Since ongoing advances will naturally erode technical barriers, disclosure is necessary in order to begin discussions while those barriers remain formidable.

When to work on risks in public vs private is a really tricky question, and it's nice to see this discussion on how this group handled it in this case.