Flutter isn't built on top of the DOM. And anyway that's not why they're not immediate mode. Take a look at some egui code and you'll see huge differences, e.g. in how button clicks are handled, when code runs, etc.
The web version has a DOM-based HTML Rendered in addition to a separate Canvas one. So (on the web, where the DOM is applicable anyway, it kinda is).
In any case, I didn't intend to go about Flutter, which I don't use, just wanted to write about React. I mentioned it while answering because the parent comment mentioned them together, and I didn't paid much attention.
The web version has a DOM-based HTML Rendered in addition to a separate Canvas one. So (on the web, where the DOM is applicable anyway, it kinda is).
In any case, I didn't intend to go about Flutter, which I don't use, just wanted to write about React. I mentioned it while answering because the parent comment mentioned them together, and I didn't paid much attention.