It does make sense. Remember that cover prevents AoO.Len said:But it also says "You can’t attack or cast spells while using your mount as cover", which makes no sense if it's instantaneous and doesn't take an action on your turn.
If it were possible to take cover from the mount while casting, you could cast in melee range without drawing AoO, without even a Concentration check. Or you could attempt a Sunder, Disarm, or other combat maneuver while denying your opponent the preemptive AoO. That's obviously too strong, so it's not allowed.