Generally I'd say there's 3 reasons for this submission working.
1. It's gimmicky and unknown what he's going to do. If you're a blue belt competitor in local competitions, you can get by with gimmicky submissions that may work on a guy once, but never again after they know you. Now he's in the UFC it may not happen again.
2. He's a specialist at that choke and you have to respect it. It's like St Preux with the von flu choke. I've hit it once during training of years in BJJ. But you have to give it to St Preux, he makes it work and is clearly a specialist at it. A lot of people may say von flu isn't high percentage. It isn't for most guys but it is for him.
3. He's way better at grappling than his opponent's. I used to spar a world class black belt whilst I was a blue belt. He sometimes joked around and would do stuff like kimura me in my own guard, tap me by holding mount in a specific way, should pressure into a choke etc. Basically if you're that much better it means you can do almost anything. Lets say I'm sparring a white belt, I can do stupid stuff which is fundamentally questionable, but if the gap is that big it doesn't matter.
Most likely it's a combination of reasons 1 & 2 in my opinion.