Nick does a nice job of sharing expertise with direct sources here. I do think leg swings are fine as the rotation in lab settings came from T9 and L1 was not statistically affected but that was with an athlete with good joint flow in the upper back. If you are bound up in the upper spine leg swings may be a problem. Nick’s youtube channel is worth sitting down and watching as he is articulate and doesn’t do anything that is dangerous. What you like and take is based on your own decision making but we can learn from anyone.