sitting in a chair requires less hip mobility than sitting on the ground. most people can get to a place where their mobility allows them to tilt their pelvis forward and then stack their spine on top in neutral position.  thus it is similar to pistol squatting to perpendicular upper leg to lower leg, or you can let your legs go a little wider and have more room to tilt forward.

if you sit with legs to each side, you still need to have enough mobility to be able to tilt your pelvis forward so that you are upright. if you sit with knees pulled up, then you’re doing something similar to the bottom position of the pistol squat which is the most challenging of squat positions.  if you cannot do either well, then you’ll inevitably start rounding your spine in order to stay upright which is bad.