A couple of points:
1) The horsenality chart doesn't say that there are only 4 horse personalities: there are 4 main types, a combination of extrovert, introvert, calm/dominating/alpha or scared/nervous/beta.
You could also say that a human is mainly one of these types (in fact I think that PP students are pinpointing their own personalities, using the 4 types.) People have been referring to "alpha" human types, or introvert humans for ages, also.
As such, it's just a helpful chart, since each horsenality requires a different skill set for optimum effectiveness. You don't treat a scared horse like you do a challenging horse, e.g.