Yes, that is true to an extent. As another example, research has found that recruiters unconsciously tend to favour more attractive candidates in job interviews. This doesn't just affect the very overweight, of course.
I think the reason most of these Fat Activists/Apologists are women is exactly because of what you said, that a woman's value is based on her attractiveness. Even though we're trying to move away from this idea, girls are still socialised to fuss over their appearance in ways boys aren't expected to, and trained to believe that their beauty is one of their most important assets. Girls are told that "beauty comes in every shape and size" more often than they're told that they can be good, talented and productive people regardless of how they look.
Hence people like our dear friend Trigglypuff here. They've been sucked big-time into the idea that beauty = merit, and they think that aggressively asserting your 'beauty' is the same thing as being empowered and confident.
It is awful behaviour, but this is basically how a lot of student activists roll. There were other people in the audience also shouting and waving their arms, IIRC. Cora seemed to be the loudest and most visible one, but rest assured, she is just continuing a time-honoured tradition of brand new adults believing they can change the world by being as loud and obnoxious as possible.
... At the risk of sounding white knight-ish, it is funny how everyone sees Cora take a swig of something and immediately decides she's pouring something fattening down her gullet. Using your voice is thirsty work, that's why speakers in a debate have glasses of water in front of them, or why choristers take a water bottle to rehearsals. So I'll give her this, at least she came properly prepared to be an ear-splitting nuisance. Maybe she should do some weights training for next time as well.