The 1DX mk III and the R6 share a sensor. The 1DX mk III costs about as much as the R3.
The distinguishing features of the R3 are its eye-autofocus, body/handling, and toughness, btw.
Just look at the feature set on the rumoured R6 II vs R3, very far from being the same.
It is a different sensor.
Every product is segmented.
The R6 is limited by rolling shutter, overheating and internal recording limit.
The R6 II will be limited by the 4k60p crop and still not great rolling shutter (pretty much just like the A7 IV)
4k120p is only presented in the R5 or upwards.
The reason why the R6 used that sensor is because it was an old sensor to being with.
The 1DX III is obsolete at this point, they offer it, but they want people to move systems.
In fact, the R6 II sensor might be cheaper to produce than the R6 sensor, hence the change.