We call this phenomenon "circular scaling".
https://cdn.**********.com/attachments/602645513648734228/875565522081693696/unknown.png
Essentially, Naruto and Sasuke closely downscale from a healthy and full power Karma Jigen, and Kashin scales to a drained Karma Jigen, operating at less than 10% of his original strength. Even if we do assume that Jigen was exactly at 10% at this point—which is objectively wrong—and that Sage Mode is a ten-times increase—which was recently bunked—he would only scale to 100% Karma Jigen, who is
5-C+.
And, why do you people keep trying to pedal the idea that "Kashin downscales from Isshiki because he 'swapped hands' with him"? I already explained that Isshiki was handling Kashin like he would a child (also, please refer to the "
base Naruto upgrade logic"), and it was further established that Kashin was completely powerless against Isshiki, something that Amado admits.
In fact, he immediately claims afterwards that Naruto and Sasuke are the only ones with the power needed to take on Isshiki, something he stated that Kashin lacks, so we have explicit confirmation that Kashin is overall weaker than Naruto and Sasuke.
With that being said, how does he scale above (
Low 5-B) two characters (
5-C+) that are directly stated to have more power than he does???