But the api is trying to simulate immediate mode using DOM

That's called an "abomination". The advantage of immediate mode is not storing a DOM. If you write immediate-mode code but it stores a DOM anyway then it's the worst of both worlds.

Isn't that literally React? I find it so immeasurably funny that in his effort to simplify code and invent immediate mode UI, Casey Muratori accidentally invented React

How would you do layouting without a tree structure in place? Not really sure what you're getting at here

You have discovered one advantage of retained-mode designs.