I agree to most of this. The team - by virtue of building and operating the software product catering for many customers - can often understand the customer usecases and solution better than any one customer. But it is the team, as a whole, where this expertise resides. Not one member of the team who happens to have a particular role and title.
> I do not see the link to the true Scotsman argument
By adding the word "effective" in:
> effective PMs know better what customers want than customers themselves.
you seemed to imply that any counterexample of a PM who is not doing this as not a true effective PM, and hence, not worth discussing.