I don't think that's a safe assumption at all.
but crop for 4k60 is always a possiblity.
the sensor still shoots at 20mp @ 20 fps - it's not exactly a slow sensor.
I still think it is, as the the 1DXIII can shoot shoot video with the full width of the sensor at 60fps.
This won't be able to do that.
So while they can both shoot the same 20fps for stills with electronic shutter, the rolling shutter is going to be worse on the EOS R6.
The question is, how much worse.
Here are some rolling shutter numbers collected from the three of the most recent Canon FF cameras in milliseconds.
('up to' means it remains the same, when it is 24p, 30p, 60p, etc.)
EOS RP
1080p FF up to 60p 11ms
UHD4K crop 33ms
EOS R
1080p FF up to 60p 14ms
UHD4K crop 28ms
1DX Mark III
FF 1080p FF up to 60p 10ms
C4K FF (5.5K RAW) 30p 28ms
C4K FF (5.5K RAW) 60p 14ms
C4K crop up to 60p 13ms
I guess the EOS R6 readout speed is going to be somewhere between the second two.
From these measurements taken, I think it quite interesting, that the 1DX III full sensor mode mode with higher frame rates have the AF disabled (Canon says due to lack of processing power) but in this mode, the rolling shutter also gets to be half as much.
I think the behaviour changes depending on the recording mode selected in-camera. So switching to MF or using MF lenses lenses won't change it, it would be quite cool, if it did, as it would become a lot closer to cinema cameras.
This indicates that for video, in some cases, disabling the AF and switching to a different processing mode seems to help with the readout speed for some reason.
So we could see it disabled on the EOS R6 as well in some modes.