NVIDIA hasn't had a competitive SoC in the mobile space since the Tegra X1. Looking at the Tegra timeline now, all subsequent SoCs have been part of their Jetson platform, or very limited 3rd party products.
So it could be a licensing issue, and they might have a successor specifically licensed to Nintendo. That would actually make sense, as Nintendo would likely want to stick to ARM, and use an exclusive chip not widely available. The discussion on Reddit is purely speculation at this point.
I do think NVIDIA hasn't been a player in this space for nearly a decade now. They were part of a great streak of devices in the early 2010s, but have effectively retired from the market for some reason. Thankfully, AMD has stepped in to fill the void.