I doubt FOIA is even a concern considering this is classified information. I think they're more worried about investigations by a future DOJ or by a future Congress since they can look at this information (if it's not deleted, that is)
FOIA requests can be made against formerly-classified information. But it's beside the point; any(?) non-classified information/communiques in government are subject to FOIA. Plenty of non-classified info in that chat and the ones we still aren't privy to.
I think the idea is they would configure Signal to delete messages after a few weeks, which bypasses FOIA because the records may not exist by the time someone requests them.
Doesn't "bypass" FOIA, it violates it. Even if the app is configured to do it for them automatically, they're actively deleting public records and violating the law by doing so.
Not all unclassified can be FOIA’d. There are exemptions to FOIA, that aren’t “it’s classified “. And there are things that aren’t “records” according to DoD records management regulations, and so would not be FOIAble, because they wouldn’t be a record to get.