I'd say your butt is tucking under slightly, but its not terrible. This is because you're going going straight back and keeping your legs too vertical. This is fine if you are a good squatter and eventually get used to it.
At some point, I'd say at the beginning of the lift you should let your knees come forward a bit more, then sit back and down.
Experiment with it and find your sweet spot. It will feel right, and stronger, when you find it.
When people say to squat you just push your hips straight back are emphasizing the wrong point, especially talking about raw squatting.
Its not a mobility issue either. With vertical shin and a big forward lean, it really does limit the depth you can acheive.