In my perspective, "Analyze code, come up with edge cases and gaps and create unit tests for them" is one of the use-cases where AI was starting to get really good at, so I can see why someone would want to extend their test-suite dramatically using it.

But yes, using AI to then generate code that still causes regressions doesn't quite square with that. Given the huge amount of test-changes I'd still assume good faith by the maintainer; possibly just a bit of overexcitement paired with a dash of too much confidence into the new tools that is now hitting reality.