Writers such as Iain Sinclair and similar who do performative walks actually build the detours round obstacles into the report of the walk as a kind of comment on land use. Entertaining and prudent.
As others have said, medical bills not an issue for UK people. Civil cases of the kind mentioned in this sub-thread don't seem to occur.
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.