This section seemed a little rushed. It's one of those things where it's like if he can take on an S-Rank already, what is even a threat to him? Hiding his power?
I think the Author sorta jumped the gun on the rematch and having the power spike even if temporarily. But that's just my personal opinion for now, maybe I'll change my mind with later content.
@Kageshinogi
Tensei Shitara Ken Deshita