I'd love to be able to do this but I can't keep my centre of gravity over my feet if I put my feet flat on the floor (no matter how widely I spread my knees), so I just end up toppling over backwards. If I do it with my back against a wall, my knees start to hurt after about 30 seconds. I just don't think my body is designed to squat like that. Or do the lotus position, for that matter.
Try to do the challenge (mentioned in the article) and go down as far as you comfortably can. You're probably lacking range in the hips or the ankles and this will fix itself with time. Basically because you weren't in this position for a long time, your body doesn't "allow" you this position, because it thinks you might hurt yourself while doing it. But once you get used to the position, you'll ease into it with time.