I think it's a bit of a stretch. The businesses reasoning was that the severe weight of the staff member would interfere with performing the job, claiming it was "a full house" and that it wouldn't be possible to walk around properly in the venue. That can easily be formalised as an OH&S concern.

As long as they did it in a professional manner, I don't think that's promoting fatphobia, which is being claimed, even though I'm pretty sure obesity is not a protected class. The fact that part of the article tries to support the case by saying the film contains fat jokes shows there's not a lot of weight to the case.

Maybe they could go for workplace bullying, but that won't work for an isolated incident.