Well I have a GTX 1080 Ti and I can get 120fps raster graphics (without RayTracing, usually maxed out settings in games released nefore 2020) in 1080p ultrawide. Maybe something is wrong with your 4090, or the games you play are unoptimized or something.

I don't doubt that the Switch 2 can actually output 120fps at 4k in some games, because if it couldn't that would be a losing lawsuit for false advertisement. But since Nintendo never really said how it achieves that, I am guessing it uses DLSS to render the actual game at a sub-1080p resolution and upscales it to do so. They said 120fps 4k, but they never said without artifacting or reduced image quality.