> all mainstream GUI frameworks, including desktop, represent their UIs with a DOM.

If you call every hierarchy of visual items with some kind of layout manager(s) a DOM, then yes. Notably, the D doesn't really apply because GUIs aren't documents, and that's exactly why HTML is kind of awkward for GUI programming: it was initially designed for documents.

Edit: Sibling comment makes the good point that the main difference is that GUIs have mutable state while documents don't. I would add that GUIs also have controls to change that mutable state, which is a more superficial difference, but well, web-based GUIs are still extremely varied in their interaction styles, which not necessarily good.