Reminds me a lot of Ben Henderson. When I lived in Phoenix I trained up at the lab and he was SO hard to choke. Apparently some peoples corroded arteries are just deeper in their neck than other peoples. So, while most peoples are close to the surface (making it easier to pinch closed), People with deeper arteries can be harder to choke. Still though, I would assume EVERYONE can get choked out. I cant imagine arteries so deep that you cant squeeze them closed. It just might feel much harder because while most people go out/tap by just squeezing an inch in, Others might require you to squeeze in two inches. Which i would guess is why it might take so long for him to tap. If you are used to squeezing only to a certain point, then you do it to someone like that.. It might not be cutting off blood flow completely like it does to anyone else in the same situation. Instead just slowing it down. Im no master or jiujitsu guru by any means.. But Id guess he just needs to be squeezed a little harder than what you might be used to doing to people. Like said previously.. sounds like a perfect candidate to practice your squeeze with.