Exactly this. You’re right about the APIs, Apple won’t accept any updates submitted using them, I think until the RC is released as that’s when the APIs are considered as RC too. Then Apple let those updates come out in the day or so before public release is iOS 16.I believe they cannot ship anything with iOS 16 APIs. But I don’t think that stops them from fixing compatibility issues that are not API-dependent. If that were not the case, none of the banking apps that didn’t work under the iOS 15.0 beta would have worked until iOS 15 was released publicly. That clearly was not the case.
Devs fixing things that are not using the new APIs in the beta can, and does happen, but they’re not obligated to do so. Also, they do not state that fix in the release notes, just generic statement about fixes.