If you can squat on the ground, why would you be unable to squat in a chair? I'm 6'1 and I've squat in my chair as well, I frankly hate sitting.
I had the same question, but given dgan's height, I think the squatting must be in place of a chair rather than actually on a chair.
I doubt that. If you're in an ATG squat, I think your desk height would need to be below your (squatted) knee height - probably well below it. I'm about 9 cm taller than dgan and doing a quick test I think a desk height of even 17 inches (43 cm) would put my arms and shoulders in an uncomfortably raised position, so realistically I'd need the desk height to be about 5-7 inches (13-18 cm) above the surface my feet are on.