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.
Pretty much this. DLSS 2.0 has been yielding some pretty incredible results.