The landing recovery has a special property which allows it to be canceled into certain actions. At the first landing frame, you can perform throws (including command throws), and on the second landing frame, you can start another jump, attack with a normal or special move, or block. Blocking not being until the second landing frame, as well as hitbox-hurtbox interactions having priority over throwbox-throw hurtbox is why sweeps work as anti airs in this game.
Things are different when jumping over your opponent, however. In that situation it's possible to walk back/forward, to jump again or to even block on the first landing frame! Strangely, if a throw is timed on the first landing frame of a jump that crosses over, a throwbox will never come out, even though that input will result in a normal coming out 2 frames later (like it normally would). Anyway, being able to block on the first landing frame can help a little bit on some safe jump crossup setups, or maybe in other situations as well if your character has a huge jump arc.