Kog'Maw, How is it possible that he have attack speed 4.3?

Hello,

I played in ranked 3 or 4 times the last days against Kog'Maw, Why is he able to get attack speed from 4.3 it should be capped on 2.5 or? I know its about his ability, but why u cant get so much attack speed with Yi's Ult or Xin's abilite + Youma? Every Champ only get 2.5 attack speed as its maximum. So why Kog'maw not?