Well, one, Roy is in the Guard, not in the mount. So yes, in the hierarchy of positional dominance, he is one up from the mount and has control of his opponents hips. So yes, indeed "possible".
However, have you ever really tried an arm bar from the guard on a fully resistant opponent? Especially one that is bigger than you? Probably not the smartest thing to attempt on the street.
I am a purple belt in BJJ and compete internationallly, I don't get these but every so often, AND we are not allowed to hit, AND we are segregated by weight class, have nice mats, gi's and are rewarded for fighting on the ground.
That said, as a teaching tool, the Arm bar from the guard is a wonderful thing to teach. It teaches you many things that are much more important than getting the arm bar itself.
This is one of the basic techniques we start teaching at the white belt level. Why? not because we expect that students will readily be able to go out on the street and use it, but because it starts developing the ability to move the hips.
Watch the video carefully and you will see that the very first thing that happens in you break spinal alignment by underhooking the leg and moving your hips off to the side. This serves to off balance your opponent (principle of irimi) and his weight is no longer going straight down on you.
More often than the arm bar, you are able to execute a sweep and put yourself in a more dominant position, which allows you to more better control the guy and possibly escape or apply chokes, or by time.
Being able to do this, however, takes a fair amount of practice to develop the skills, the attack chains, and the timing to be able to execute jiujitsu from the guard. It is why we BJJers spend so much time in the Guard.
From the guard for a self defense perspective, I would tend to teach cross collar chokes, strikes, shrimping skills, and teach them to stand up in base once they created distance, way before i'd spend time on teaching the Arm Bar from the Guard as a method of self defense.
Again, it is a good base technique for learning JJ fundamentals, and is an excellent foundation to build attack chains upon. As a money maker....not likely for a female being over powered by a guy...too technical and way too much risk involved, and there are many other things that should be taught first that are more "high percentage" and safer.
Wrist locks, arm bars and such might be useful in buying you time or creating a space in the fight, however they don't typically end a "real" fight or rape, they tend to piss the guy off more.
However, a good blood choke executed shuts things down. I'd put my money on establishing a position that I could execute a blood choke way before ever using an arm bar.
It was just one scenario, but surely not the only solution. Eye gouging, biting are always options. Maybe it want work. You have to try if it will help save you. The possibility is there for one who knows how to execute the technique. Yes it takes some skill. Learning these techniques in Aikido and BJJ both take time and a lot of practice.
In my Aikido dojo we have a black belt BJJ instructor teach us basic techniques. I have by incident pull off an arm bar in our dojo against an unruly student who became angry and wanted to challenge me. I was in the guard and just reacted, it worked. Could I do it again with precision exactly, maybe not. His arm was just there, saying lock the hell out of me. His reactions were pretty slow. Perhaps that was more of the reason I was able to execute the arm bar. Would I try that on the street, probably not. Would I try this if I were in a grassy area or dirt field, maybe. A person has to know their surroundings and learn what's applicable during a particular altercation. Self defense training for every possible situation takes years, maybe never. The key is to make you mind and body accustom to conflict trauma. This will at least hopefully give you the advantage in an altercation.
I've been training in Aikido for about two years. BJJ for almost 7 months. I'm new to both martial arts. Prior to Aikido or BJJ, I trained in Burmese Bando for 5 years reaching a brown belt. Bando is the sister to Muy Thai. Bando has grappling also and we learned chokes and arms locks, but applied differently. Martial Arts isn't new to me. I'm pretty athletic and have a natural deposition for the arts. I learn fast.
Having the knowledge of physical altercations and being familiar with body trauma, prevents unfavorable emotional reactions. A person needs to use their head and think as quickly as possible. Our BJJ instructor always tells the female students that this technique may help you in an assault. It's no guarantee.