AFO hasn't been shown using any defensive quirks. Not even Impact Recoil which could have easily negged Mt Lady, Tokoyami, and Gigantomachia.
I think rather than "mountain level swords", "mountain level syringe

" or the assumption that AFO's helmet is somehow more durable than his Prime self, it's pretty obvious that post-rewind, AFO wasn't as concerned with defence.
For someone whose stats come from whatever quirks he activates, one can't just assume he is at max level durability.
Same way we can't assume each of his attacks is max level especially when each attack involves different quirks or different combinations of quirks.
Those quirks or combinations would never all have the same power same way the people they were taken from didn't have the same power levels.