Ironically, avoiding assumptions is the basis of product management.
At the end of the day, someone has to do the product role on the team. If the founder can scale that, great, and if the engineers can do it, even better! Unfortunately both options are extremely rare to find.
There are a lot of shitty PMs out there, just as there are a lot of shitty engineers or any other job function.. We would do better, I think, to discuss and promote better patterns, than trying to discredit a whole profession by clumsily reverse engineering a handful of gross assumptions and, probably, bad personal experiences.