Sorry, don't believe this Redditor on the basis that there really is no logical reason for this limitation of 4k30. Like, even a simple 2D game can't even hit 4k60? It's more believable to think if this person did see it in action, he saw a game running at 4k30, not that the system was limited to 4k30.
Seriously, every single Nintendo platform before this, both home and portable console, had a top resolution that also hit 60hz. Not every game pushed 60fps, but the hardware was capable of it. And with Switch 2, it will have the benefit of the superior DLSS upscaling for GPU-bound games to improve frame rates. You don't throw in an impressive Cortex A78C CPU into the system, allow portable mode to push 1080p60, and then go on to say docked mode sucks because 4k can only be done at 30hz.
It makes even less sense when you start to think about backwards compatibility and the possibility that Switch games could be enhanced on Switch 2 to make use of the stronger hardware to go beyond Switch's 1080p60 docked mode limitation. MK8D, on hardware that could easily run it at 4k60, can't because of this hard limitation? How about NSO with 3D-rendered games running under emulation?