As someone who lives not far from this very hot place, I can assure you that it's exceedingly popular, especially with international tourists. Brits, in particular, love the desert and flock into Death Valley during the hot months. Some write glowing trip reports on TripAdvisor (those are the ones who follow the rules, I guess). Many do not write reports, though, and I always wonder about them.
It's on a continuum with people who go in privately built, non-certified submersibles, I think. A kind of wager with oneself regarding adventurousness and courage. China has some really hot places, too (and many are humid - they say that Morongo Valley is "not so bad" when it's 155F - and by "they" I mean "people I know who live out that way.") SO many hot places in inland California. 102F at 10 am in Borrego Springs (not far from Julian). Going up to only 106F though - so a good day to go out! (It's going to be 109-110F at my workplace today, supposedly)
Here's the thing. People stay in their trucks and cars, with A/C. They jump out to take pictures. They go into restaurants and hotels that have A/C. Death Valley is pretty much closed right now (flood damage) but there are signs everywhere in summer saying things like "Do not be out of your car for more than 5 minutes."
Joshua Tree is already 100F this fine morning, already hotter than predicted. Only 95F in Morongo Valley. Anyway, millions of SoCal people will endure temps over 95 today and think it's pretty "cool" until it gets to 112-115F.
Only the coastal zone escapes this in SoCal. And AZ is often even hotter (and people flock there - including to live there). I'm sure JRF must have A/C at his place, I'd bet on it. So a long day popping in and out of the car...and so far, no evidence of actual camping (which would indeed risk heat exhaustion in many places, this time of year - but not so bad if they are up at Big Bear or in the Julian outback).
I just think one of them would have checked in by now. Oh, and this is not "one of the hottest summers" here in SoCal - we could still earn that as it's usually hotter in September than it is in August or July (even inland) and those conditions are predicted for the next 10 days. So far, though, many of us living in less than record heat.
IMO.