I suspect the GP saw this during a point release update on Mojave. Baseless speculation: I suspect that they were referring to Apple making use of the ability

I suspect the GP saw this during a point release update on Mojave. Baseless speculation: I suspect that they were referring to Apple making use of the ability to spin up a new temporary APFS volume to store the Update packages + Staging environment and blessing it for boot. It makes a certain amount of sense to push out a static staging environment to then run the requisite OS updates and firmware updates from that controlled environment. That way you can reboot as many times as is required for the updates in the point release and still come back to the staging environment ready to go to the next stage - only needing to touch the User Data Volume when absolutely necessary. While not nearly as 'clean' as an atomic update on a read only root filesystem, this method could be significantly more controllable and more 'clean' than doing it live on the User's Data Volume. I suspect with iOS Software updates - it's a transactional atomic update as those machines all use read only root filesystems regardless.