Assuming that you are referring to the Nreal Air VR goggles, those cannot accept a regular video signal over USB-C. They are NOT monitors. For making them work, they need to do pre-processing of the video stream on the host device, thereby degrading gaming performance. That's why they need an app, because they lack an integrated CPU/GPU and that makes them not compliant with the USB-C monitor spec.

And I believe that's also the real reason why Nintendo would block them: They make otherwise fluent games stutter, which will reflect badly on Nintendo and the game developers. Because people in general won't know that their Nreal Air is stealing some of the Switch's GPU performance and that they are creating performance issues themselves.