I think of pigenonholing as a cognitive heuristic that spares you to involve your "system two" [1] when dealing with or talking about other people. It is a heuristic insofar as peoples' behaviour is more like a continuous spectrum than a discrete set of categories.

The other cognitive heuristic that applies here is that of projecting n independent dimensions onto one or two, at best. Like in this case, the latter often comes with exactly two elements in each dimension yielding a 2⨯2 matrix.

[1] https://en.wikipedia.org/wiki/Thinking,_Fast_and_Slow#Two_sy... (admittedly a pigeonholing of mental processes by itself)