This is effectively a claim that Mozillia, Apple, Google's developers are idiots and irresponsible and just accepted the fixes without checking them. That's a pretty strong claim which I think you'd have a hard time proving or even finding any evidence for

I think LLMs are particularly effective at turning once-competent developers into idiots. (“Just ask Claude.”)

I mean no such claim. One does not have to be an idiot or irresponsible to overlook an unintended consequence of some code change in a very large, legacy code base.