Sometimes it's better to light a flamethrower than curse the darkness

What makes it stranger is that ticking the box just expands the slider into a larger set of possible positions. It doesn't add additional menus or generate more boxes. There's no additional cognitive load. It might become more difficult for persons who have motor disabilities, but the slider itself was going to be an issue for them. So I think you might be right about the tickybox being about abled comfort and the belief that someone isn't really truly disabled if they need some text magnification for reading. (Coming from someone who doesn't need glasses, but who often finds bumping up the zoom when looking at big screens or tiny typeface sizes much more comfortable and who likes having the subtitles on, even if I theoretically don't "need" them. I'm not disabled by social standards, but damn right I will take advantage of those kerb cuts.)