From what we know, this doesn't seem like a "walk-away" to me. folks like Paul Miller (responsible, employed, family he's responsible for, "solid" sister, etc) don't intentionally disappear very often. Esp a Canadian in the US. Likely, he went off trail to take a photo-chasing bighorn or the like. Got turned around. then twisted an ankle-fell, or the like...
I know the park, a bit, but haven't hiked this trail. Don't see an "obvious" goal off trail which would have led him away, but...
Folks die in J'tree (true of most NPs, but this one is particularly unforgiving). A few years back, a couple (Dutchman and a German) were looking for "the" Joshua Tree which was photographed for the cover of the U2 album "Joshua Tree" . Sadly, it's nowhere near J Tree NP.
Guus Van Hove, Dutch Music Man Who Died in Joshua Tree, May Have Been Searching for Site of U2's Album Cover