The main thing that confuses me is that this seems to be PHP implemented in React...and talks about how to render the first page without a waterfall and all that makes sense, but the main issue with PHP was that reactivity was much harder. I didnt see / I dont understand how this deals with that.

When you have a post with a like button and the user presses the like button, how do the like button props update? I assume that it would be a REST request to update the like model. You could make the like button refetch the like view model when the button is clicked, but then how do you tie that back to all the other UI elements that need to update as a result? E.g. what if the UI designer wants to put a highlight around posts which have been liked?

On the server, you've already lost the state of the client after that first render, so doing some sort of reverse dependency trail seems fragile. So the only option would be to have the client do it, but then you're back to the waterfall (unless you somehow know the entire state of the client on the server for the server to be able to fully re-render the sub-tree, and what if multiple separate subtrees are involved in this?). I suppose that it is do-able if there exists NO client side state, but it still seems difficult. Am I missing something?