I notice you edited your comment, so I'll reply.
Thank you for your answer that it does infact replace Classic HD models and textures, as well as SD ones.
Since you say you're trying to help, I'll give you the benefit of the doubt that you genuinely didn't understand why I was so frustrated by your responses. In general, if someone says they read X but didn't understand it and would like an explanation of what X means, and all you do is screenshot X with no added explanation of your own, in normal situations (e.g., in the workplace, or even when texting a friend) that is considered rude, unhelpful, and maybe even a bit condescending. Likewise, a response like "it says blah" is rude compared to saying "it supports blah," because the former assumes I didn't bother to read the thing we're discussing despite saying I did. If I said I read it, then I already know what it literally says; I don't need it merely quoted back to me. Does that make sense?
Now, to elaborate on why I didn't understand the webpage: because it said SD was suggested, it wasn't clear to me whether it actually changed anything with HD, or whether it merely didn't break the existing Classic HD graphics.
The text itself didn't use the term HD ever, so I only had the settings screenshot to go off of, and I wasn't sure whether that was just a before-after like what I assumed their other screenshots were, so I wasn't sure which setting the mod was actually primarily design for.
In particular, I'm looking for some kind of HD models, so the talk about SD on the site was (and tbh still is) confusing - that's why in my original question, I asked "why" it would be for SD (ie, what does it even change about SD? Just things like the possession animation?).