I agree with this. In any given team, the PM should be the least expert on any given subject (e.g. what parts of the codebase to utilize, how to design the page

I agree with this. In any given team, the PM should be the least expert on any given subject (e.g. what parts of the codebase to utilize, how to design the page, etc.), otherwise you've staffed the team incorrectly. Forcing the PM to create requirements based on things she doesn't understand well is a surefire recipe for failure, as you said.