Actually not necessarily true, there are about 20 cultures mostly in South American that practice partible paternity, where usually two (but sometimes more) men are considered the father of a child (because the woman had sex with them also at the time she got pregnant). Those kids have about a 15% higher chance of making it out of childhood alive.

But, yes to the rest of this. If mediocre men are worried about not being able to kind of coerce women to get with them due to lack of mate scarcity, they should up their game. Actual understanding of how consensual non-monogamy works dispels all these silly ideas, as you pointed out.