If some number of kids watch his videos and get hurt mimicking them, he'll accrue a bunch of lawsuits to cover their medical bills.
Putting the disclaimer implies nothing but a well-founded fear of frivolous lawsuits.
Now he did put a disclaimer on one of his videos. But in many other videos he’s been talking about this kind of mission as if it might be something that the viewer might wanna do herself. “If you’re planning a straight-line mission…” Stuff like that. So he might not have been encouraging it, but he has been talking as if his videos might inspire the viewer to do them themselves.