The biggest difference between Alex and Mahomes besides the far superior arm is the instinctive pocket presence and ability to keep the head up and going through progressions. Alex was awful at it, PM15 is elite.
That said he’s not wrong Alex would be the second best QB in this division. But in fairness two of the three teams have 22 year old unproven guys with superior arms but have proven nothing. Carr is indeed Alex minus the running ability.