I still feel like there is a mistake, or more to this. Polearm proficiency is useless. Classes that might make good use from polearms already get the proficiency. Classes that don't get a polearm proficiency aren't improved by having it.

A couple of possibilities:
1) Fextralife misunderstood, and humans can actually choose a weapon proficiency from a list (and the list happened to default to polearm)

2) Fextralife misunderstood, and humans get expertise (double proficiency) in polearms

3) Larian has Homebrewed Polearms (or just Q-staffs) to add finesse to them.

Or there is some other unknown factor that makes this make sense.