Pointing out to someone that they shouldn’t make fun of potentially mentally disabled people hardly equates to being on a high horse.
err ...
* Lambchops all but puts his neck out trying to look up at swift
... I'd say it literally equates to being on a high-horse.
Whether or not a point is valid is not a factor in high-horsery. Surely believing oneself to have the moral high-ground is in fact a pre-requsite? At any rate I'm guessing it's a metaphore that both you an BS have heard more than a few times.