To explain the
High 6-A scaling yet again for any staff just looking in.
Izuku is vastly below 100% when he performs that feat. He's not using
100% as you
can tell by his appearance, and
he's severely weakened by his injuries. Izuku's Full Cowl is slow enough that
he needs help from others to avoid AFO's attacks. Kurogiri is also shown to intercept him
mid High 6-A punch as well.
Kurogiri is slower than Weakened All Might. This means Izuku is slower than Weakened All Might as well.
When using Full Cowl, Izuku has never been shown to increase his power and speed independently. Here's an example to help explain, Izuku cannot use 10% Full Cowl for speed while his power is at 20%. If his speed is at 10%, that means his power is at 10% as well. The only exception would be if Izuku was mixing percentages in different limbs. Like if he was using 10% in his legs to run and activated 20% when he threw a punch. However, nothing in the final battle states or even implies he's doing that.
Without proof, that's just meaningless speculation. Also, that's contradicted by Kurogiri intercepting Izuku mid punch as shown above. If Izuku's speed is below Weakened All Might, that means his power is below Weakened All Might as well. As such, Weakened All Might should scale to the
High 6-A feat.
I also think it should scale back to Izuku during the You're Next movie. Izuku is shown to struggle against Dark Might, and at times is even desperate as well. Implying he's giving it everything he's got that isn't 100%. Izuku also performed that
High 6-A feat without 100% and was severely tired and weakened.
While in the movie, Izuku is completely fresh when facing Dark Might. Full Cowl Izuku in the movie should be stronger than the heavily weakened Full Cowl Izuku in the Final War Arc that happens three days after the movie. There's no reason to assume that an Izuku
that can barely walk, is stronger than a healthy Izuku.
Izuku's statement here is also not about his current power in this moment. It's about the power of One For All itself, which he just transferred into Tomura's body. That's the reason why we see a flashback of All Might telling Izuku his current body cannot withstand OFA's power. What All Might said to Izuku is happening to Tomura. His weakened body couldn't withstand the power of all nine of them being forcibly transferred into him, and is falling apart as a result.