I think the fact they are pushing off of an elevated surface makes it stand out with some of those pitchers. There are some of the pitchers in that video, however, that are able to keep the drag foot on the ground so we can't say for certain that it is the device.
I've been on that site and it appears that the girls in the video are new to using the device, so it is hard to say if their drag foot problems are new or (more likely) old problems. That is the problem with using players to demonstrate a product when they are just being introduced to it.
I just recently came into possession of one of these devices, but haven't had much opportunity to try it out for pitching yet (basketball season). I'll try it out this week and report back.
We did try it out again this weekend for hitting and I really like using it for the stride foot. My DD tends to open up a little to early and this helped out with that.