There is nothing in Rule 8 that makes this illegal. I have never seen this before. Amazing.
The rule about one step back and one step forward is focused on the free foot. His free foot only strikes the ground one time (one step forward) and he keeps the ball low to delay his arm long enough for his lower half to do its thing.
About the post foot, by rule, it only has to start in contact with the rubber. He just puts his back down early. Some people drag, some keep it off the ground, and apparently this guy puts it back down. He’s clearly not attempting a pick off and it’s also clear he intends to deliver to the plate. As long as he does it every time, it should be legal. If he only did it on certain pitches, we might be able to have a discussion about a balk, but I probably wouldn’t call one. It’s really no more deception, albeit, a lot less common than a pitcher changing arm slots for various pitches or switching to throw left or right handed. It’s strange, but I’d say it’s legal.