You'd be right if you are talking about a beginner. He's not. He's been doing this longer than some of the posters here have been alive, doing work in a lower stance helps you learn to develop balance in a taller stance.
Also, he's actually not that low to the ground, here look at this:
Bazooka is doing the same thing here, and this guy was the Glory middleweight champion and employed a fairly dutch style. What
@shincheckin is doing is completely fine even from a kickboxing stand point, he is simply allowing himself to get leverage into his punches, he probably won't stand that low in an actual fight, if he were doing it all the time even lower I'd agree there would be a bit of an issue, but for this to be one specific exercise, I don't think it's enough to cause any significant problems, it's not like he's shadowboxing and trying to throw high kicks from a low stance