If the Series S can't run games at native 4K why would a Switch 2 be able to?
Only if it's straight-up 1080p upscalling, which the system would do, not the devs.
Nvidia DLSS could do it because you only need to hit 900p-1080p to get 4K. So if Nvidia is giving them a chip with Tensor cores, there's your answer right there.
Also running Mario Kart 8 (a game that already runs at 1080p on the current Switch) at 4K is much, much different from running say ... a next-gen game at 4K.
You can't just look at 4K as one standard metric.