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.

[1]: https://pthom.github.io/imgui_explorer/

[2]: https://github.com/ocornut/imgui

With that argument, every UI is retained mode because everything gets saved in the GPU's scan out buffer.