On the flip side, my old PM had an incredibly deep understanding of both the product and the customers buying it (an area in which us SWEs were lacking), in addition to a semi-hands-on understanding of the engineering process too.

He wasn't a professional software engineer, but he took the time to actually learn how to program through some side projects and through this gained a real understanding of what would be trivial for us to implement and what wouldn't.

To top it all off, he had good rapport with the CEO and was adept at managing upwards.

The product wouldn't be half as good as it was if not for him.

Rather than tarring an entire profession with the same brush, I think a much fairer "warning" to founders is to make sure the people you're hiring are competent and that the job they are doing is something that actually adds value to the organisation. PMs, when deployed appropriately, can be worth their weight in gold.