He had two phones with him when he vanished. If he ran out of water, or lost his way, and he had his wits about him, he could have phoned for help. He did not.
That suggests that either something happened so fast that he could not use either phone, or he lost his wits and didn't think to use a phone.
I'm curious - if someone is dehydrated, or suffering sun-stroke, are they prone to taking risks? If so, that would account for going a few feet off trail, if he went a few feet off trail, wandering around without using a phone for help, lost wits, or does dehydrated mean keeping our wits about us and calling for help?
I'm leaning towards going a few feet off trail and slipping. Was he wearing proper footware?
He sounds sensible enough to be wearing footwear with a decent tread and possibly some ankle support. Previous hikers report some areas have rocks and gravel, but it's a very well traveled path.
Is there a way to triangulate a location of either of the phones?
My impression is that someone suffering from heatstroke is going to head for any kind of shade they can find, or think they are finding. He could have tried to climb underneath a ledge or rocks and not be seen from the air.