Riot rewrote his lore confirming he's not a yordle. They just haven't patched it in yet. It's confirmed by tons of redposts in forums but i cba to go look it up right now cause i'm going to bed. But yeah, as dumb as it sounds, the lore is incorrect. He's not a yordle.
PLEASE SEND SOURCE TO ME ASAP
THIS IS A HUGE PEEVE OF MINE
AMUMU IS NOTHING YORDLE-LY EXCEPT BEING SMALL AND FOR ALL WE KNOW HES JUST A KID