Hypothetical:

Let's say Ezren is a level 20 wizard and has wish prepared. Now, Ezren encounters a pit fiend, a level 20 creature. Ezren decides he doesn't want to deal with that and casts wish and uses it to replicate the spell banishment. The pit fiend gets a (non-critical) failure on its save against the wish-turned-banishment. Now for the question. What happens? Does the pit fiend improve its degree of success?

Banishment has the incapacitation trait, and wish duplicates the effect of "any spell from the arcane list of 9th level or lower to which you have access." So...is it 1) a 9th-level banishment that's basically using a 10th-level slot? Or is it for some reason 2) a 10th-level banishment. My gut tells me the answer's 1), because that seems most obvious, but I'm not 100% sure, so I thought I'd ask.