Oh, I agree completely with you, sorry if that wasn't clear. The PM should, must, care about UX. Still, they don't always do, or at least end up not caring eventually, for various reasons.

I'm just responding to this:

> what were your Product Managers doing in the first place if tech writer is finding out about usability problems

They might very well be doing their job of caring about UX, by using the available expertise to find problems.

It's a bit like saying (forgive the imperfect analogy): what are the developers doing talking about corner cases in the business logic, isn't the PM doing their job?

Yes, they are. They are using the combined expertise in the team.

Let's allow the PMs to rely on the knowledge and insights of other people, shall we? Their job already isn't easy, even (or especially) if they care.