My lawnmower (openmower) can do <2cm accuracy over GNSS. It is absolutely believable they could achieve 0.5cm on GNSS (plus rtk correction data from a fixed base station nearby) alone without measuring any relative distance using other systems.
Well, it isn't "uncorrected". It's just that you don't need any additional hardware other than a second gnss receiver on the base station and some kind of link between them.
GNSS is more than accurate enough once you know all slight errors in satellite orbits and the atmospheric distortions currently affecting the area near the base station and can correct for them.
