The performance of the PS5's GPU is at least 84.6% of the performance of the XSX's GPU; I say "at least" because the cache scrubbers and the Geometry Engine of the PS5's GPU offload certain tasks from the components of the GPU that are used to quantify performance in terms of teraflops.
(10.28 teraflops / 12.15 teraflops) x 100 = 84.6%
So, if the XSX will be able to run this Dolphin game at 120fps, then the PS5 will be able to run it at at least 101 frames per second if the graphics settinga are identical on both machines. Hence, by merely decreasing the graphics settings a bit on the PS5's version, it should be able to hit 120fps. Having said this, I don't think that this dolphin game is demanding enough to require the settings of the PS5's version to be lowered in order for it to hit 120fps; I think that this developer is spouting bollocks.