I want you to know Izzy does know how to check leg kicks. He knows how to roll the leg aside or back too. The best leg kicks are not telegraphed and just a few of them can cripple a man for the rest of the fight. If someone lands some jabs on you, does that mean you have terrible defence against jabs? No, not necessarily. Just like calf kicks, the best jabs aren't telegraphed. Even the best fighters don't see every shot coming or can't defend against every shot coming. Even the best fighters have to weigh their options and prioritize their offence and defence. To stand wide, to put weight on the lead leg or not. To allow a calf kick through, in order to land counter strikes upstairs.
A valid criticism of Adesanya in those fights would be that he got preoccupied in a leg kicking battle, going tit-for-tat, which made him predictable, interrupted his pace and his game plan. He made adjustments to that in the 2nd UFC Periera fight.
Saying he's "terrible at defending leg kicks" is just a bad take.