I am not sure why they think that is true, but I can assure you that isn't an actual concern. While it might be possible to let a bay do this in a warm climate, you can't even consider this is cold climates. Just a generation ago for me, my parents grew up with frost on the inside of windows in winter. You can't very well leave a baby with a naked bottom when the indoor temperatures are enough to cause you hypothermia. Even now, with better heat & insulation, I can't imagine that my daughter, born when it was around 0F/-17C, was going to sleep for me at all without warm clothing on. I think what we call air time is good. It's just diaperless time relatively often to allow the skin to dry completely, etc. We tried to do that when we could & I would use a space heater to warm out bathroom & allow them to lay on towels on the floor, but I can't see being able to do that all night.
I think if this were truly a concern orthopedic Drs would be speaking up about the issue. They checked my small baby's right after birth & a few times after for hip displasia & they do look at how hips & legs are coming along after birth. I would think if anyone had a concern, it would be more about hips than legs. I am not sure where the concern about legs is coming from or if perhaps hip diplasia is what they were thinking of, but did know the proper name.
Here is a whole article on diapers, hips, etc that may ease your mind. https://consciousbabyblog.wordpress.com/2011/11/27/cloth-or-disposable-how-diapers-affect-your-babys-development/