Well if we are trying to break it down from the video (even though we can't see his left hand) it appears that there are a couple of additional differences in technique from a regular triangle.
1 Romulo never shrimps/scoots toward his right but stays facing straght on to his opponent
2 Romulo doesn't cinch his right shin ALL the way behind the left knee so it is a little more like a grapevine than a figure 4 of the leg.
The combination of 1 and 2 allows Romulo to put pressure from his right thigh (just above his knee) into opponents neck.
3 Not sure if it is critical but Romulo rolls toward his left side just before the tap. my guess is, this, combined with whatever Romulo is doing with the left arm increases the pressure on opponents neck on that side.
4. If Romulo's left arm isn't inside the collar where is it and what is it doing? I Can't see squat in the video but after the tap when Romulo releases his grips and legs, you can see his left arm retract. This looks like Romulo was pulling hard with his left arm and was holding something with his left hand and when he released the grip with the left hand the left arm pulled back momentarily. This indicates he is not using his left arm to push his left leg into the opponent's neck but is pulling on something. My guess is he is pulling on the opponent's sleeve, trying to drive the opponent's own upper arm/shoulder into the outside of his left knee. This also prevents the opponent from using that arm to try and push out of the triangle.