It's not an assumption, he's literally shown performing against Hidari like you mentioned, a feat he wouldn't be able to perform by his previous showings, statements or narrative.
You still need a reasoning to put him in Low Kage in the first place btw. Else it would be the equivalent of "Y is relative to X who is featless so Y is trash".
Oversimplifying years of training to the Jutsu they gain allows me to say "Zabuza > Base Naruto > Pain, he only got more Rasengan so he's like the same as in P1". It's really not good.