There already were legendary players (rebounders) halfway season 4. There even was 1 player with skill 21. Dunno if that's fixed already.
I think 21 is a "point skill", the same way 0 is at the other end of the spectrum. That is, the maximum skill level in the game is 21.0 -- this is speculation, though.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."