No, your understanding of the term is correct and I agree with you. I think the confusion comes more from the tumblr post than anything else. The post says she is being discriminated against because people don't like her because she's different, but it also states that she is being discriminated against because she is disabled. However, the post doesn't identify Sarah's disability, it just states that she is disabled. Likewise, the game doesn't distinctly say that she has a disability, and also doesn't seem to suggest that she has one either. Abelism is a form of discrimination that is distinctly targeted at people with disabilities, not just people who are different in some way. In that context, I think the post is actually insensitive of people who actually have a disability. My younger brother has Down Syndrome, so maybe I'm a bit more sensitive to the topic, but I think it's wrong to label someone as disabled if they don't actually have a disability, which seems to be the case here.