Actually with her positioning (perpendicular to the right shoulder), she gets adequate leverage and angle to pull her right forearm into the neck while using her other arm to add extra leverage by pushing the head away.
The mechanics are the same as the Darce/Brabo. You aren't trying to drive the shoulder down into the neck but rather you are pulling your arm up into their neck. You don't need to choke off both sides.
When done in the position that Kim is in, the problem would be in the girl simply spinning out of it counter-clockwise. Left foot posted to the left hip or get your right hook in to prevent the escape. If she spins the other way (clockwise), she turns right into a very deep arm triangle.
Garbage move? Not really.
Low percentage? Only when you give them the easy escape.
I am not trying to be rude, and people will tell you I'm the friendliest person ever, but you most definitely DO need to cut off both carotids for the Darce.
There are 3 pressures needed to have a proper Darce: let's pretend a right armed.
1. Your right forearm cutting into their right carotid artery.
2. Your chest/shoulder against their shoulder, which cuts off their left carotid artery.
3. The support arm, the backstop of the arm triangle, which keeps their chin tucked low, pulling their head and neck down into the V
You can finish the Darce/Brabo from top side, top half, off your side cloking in, from the mount, doesn't matter. As long as you have those three pressures, you will finish. You can lift your elbow, dip a shoulder, elbows together, spread your back, etc, etc, But you NEED those 3 pressures. You may finish someone with 2 out of the 3, but it's not good technique.
I could go through lists of the needed pressures with every head-and-arm variation: Anaconda, arm triangle, reverse arm triangle, Peruvian necktie, and so on, but you get the point.
You will never be able to go up against any good grappler and finish a head-and-arm choke or even blood choke by only cutting off one artery. They will not tap or go to sleep and they will escape.
Back to the move in question: It may not be garbage, but it IS low percentage. Evidence tells us it is, being that it has only been done one time at the upper levels of competition as far as I know, and it was done by the current best no-gi grappler on the planet. It's very simple. If we actually saw this even on a SEMI-regular basis no one would be calling it garbage. I AM not saying it's garbage, but it is most certainly low percentage.