While you’re correct that GNSS is not 100% accurate (like everything in the physical world, it comes with error bars), I think the point stands that the error is unrelated to the movement of continents.

Ultimately GNSS is just measuring your position relative to some celestial objects using the time it takes for signals to propagate space (and some other info: their ephemerides, and a shared-ish clock). The fact that land exists, and where it exists, are not relevant

Your error corrections might be thrown off when the base station moves, but that’s not an issue of GNSS that’s an issue of RTK