There are at least witnesses who saw the box of treasure before he supposedly hid it. (He kept it for a few years after his cancer went into remission.) Also, he donates all proceeds from the book with the clues. And everyone who knows him says that, yes, this is exactly the sort of melodramatic stunt he’d pull. For instance, Fenn also casts bronze bells with messages to ring it for him if found, and buries them all over the American west. He likes the idea of being remembered (which admittedly a hoax would also accomplish) but the treasure supposedly includes a copy of his memoirs. He’s an eccentric guy. He also seems semi-responsible about it: for instance when people started digging up old graves he specified that it is not in a grave, and when people got in trouble by breaking into bathrooms in national parks he specified that “warm waters halt” has nothing to do with a bathroom, etc.
The big issue of course is if someone who finds it can even legally claim it. If it’s on public land it’s almost certainly illegal to remove it (it would be an artifact), and if it’s on private land it belongs to the property owner. There are legal arguments to be made, of course- for instance that the treasure wasn’t “abandoned” in a legal sense, but rather left for whomever finds it, etc. And believe it or not there is a legal definition of “treasure”, and it does imply “finders keepers” to a certain extent, depending upon the state. Nonetheless, the best option for whomever finds may be to never ever say where they found it…
I included checking out a Fenn solve on our trip to Yellowstone last year. If nothing else, it gives you ideas of where to hike:

In retrospect it wasn’t a good solve- there’s no way an 80-year-old could have hiked to it from his car twice in one day (which is another clue he has given).
Even if it is a hoax I think it’s a great idea to get more people outdoors. Not to mention that it’s just FUN, dammit! But I guess I don’t think it is a hoax.