Well, people also say "oh I want to take him out because he is eating some of the eggs". Except that, if it isn't the male's first time and he has had successful spawns before, he may be eating the eggs that are infertile or infected with something.
I think the experiment has a point, because fish do know more than we will, especially about their own kind!! :) And plus, I will note with balloon mollies especially... separating the fry, causes lazy fry. Keeping them with the parents, with floating plants and such create fast growing, stronger fish - clear of deformities (usually eaten)