I do understand that's what they have in mind. But that's not what immediate mode means. Function application is not the only criteria.
We are talking about rendering into a browser by twiddling DOM APIs. That's definitely retained mode rendering and repeatedly claiming it's immediate mode doesn't make it true
If you compare the code using the presented framework to code using say immediate mode library like ImGui[1][2] for example, the presented framework does indeed seem to be exposing a retained mode API.
You keep using that word. I do not think it means what you think it means.
React most definitely falls under what would be called a "retained mode" of some sort. And the UI library described here is also.