The drill shown in that video is a good way to get a temporary increase in your ankle range. A temporary increase.
Most mobility drills give you a temporary increase and not a permanent increase.
That means that a piece of the puzzling is missing.
The butt wink is typically thought of as the low back rounding during the bottom of the squat. Now, while that IS true (the low back does round during the butt wink), there actually is a purpose to the butt wink.
The ankle joint for many people is pretty stiff and could use a bit more motion. I am not a huge proponent of banded joint mobilizations or self joint mobilizations with external loading, BUT the ankle is a joint where I think the benefits are large and the risks are very minimal (aggressive banded mobilizations of the shoulder may not be so good for everyone, for example).