In this sequence the model is pulled to their toes with the waist line as part of the second step. If you pull them to their toes with the box tie, you will lose some of the aesthetics of this and subsequent positions. That doesn’t mean it’s the only way to tie a kata ashi though! It’s just what makes is recommended to be able to follow along with this particular sequence.
As for your question about verticality, the distance between the thigh line and torso as well as the bottom ankle cuff are all contributing to that angle that you’re noticing, rather than the height of the TK in the beginning. Bringing the thigh closer to the upper body and the ankle more below the hips would create a more vertical position (which many bottoms would likely find to be a bit more strenuous).
Replied on Bunny Tie | Tutorial
18 Jul 16:24
Adding a little friction before the cinch will help to maintain an even tension within the cuff and assures that the cinch is isolated from the cuff. We encourage thoughtful exploration though! If you experiment with placing the cinch before the building the friction, make sure to isolate the cinch from the cuff.