RTK is really about correcting for momentary atmospheric conditions that lead to SLIGHTLY different propagation times for radio waves in order to get much more accurate measurements. The idea is that you take nearby fixed objects that are constantly recording their GPS position, and so over time their precision in their GPS fix goes up. When you hop on with your RTK, it will communicate with those nearby GPS stations and apply a correction based on what they measure vs. the position they know they're actually at.
What you're talking about is totally true though. And it's why defining land boundaries by GPS coordinates is not a good idea.
That's when we get into the wonderful world of coordinate systems. These are deeply involved with RTK, and not knowing about them can mean you get incorrect results even with the RTK, most correction services actually use specific localized coordinate systems, and even offer additional correction data when even that is not enough.
What you're talking about is totally true though. And it's why defining land boundaries by GPS coordinates is not a good idea.