Earlier quoted context omitted.
You're making the opposite mistake: you're seeing someone's description of a tool's feature and confusing it with the way we've already done things without even checking if the tool is transformative just because it kinda sounds similar. Astro's main value prop is that it integrates with JS frameworks, let's them handle subtrees of the HTML, renders their initial state as a string, and then hydrates them on the clien…
What is the value in first sending dysfunctional HTML and then fixing it with later executed JS? If you do that, you might as well do 100% JS. Probably would simplify things in the framework. Sending functional HTML, and then only doing dynamic things dynamically, that's where the value is for web _apps_. So if what you point out is the value proposition for Astro, then I am not getting it, and don't see its value.
Just compare the two cases, assuming 100ms for the initial HTML loading and 200ms for JS loading and processing.
With full JS, you don't see anything for 300ms, the form does not exists (300ms is a noticeable delay for the human eye).
With frameworks such as Astro, after 100ms you already see the form. By the time you move the mouse and/or start interacting with it, the JS will probably be ready (because 200ms is almost instant in the context of starting an interaction).
This is not new at all, old school server side processing always did this. The advantage is writing the component only once, in one framework (React/vue/whatev). The server-client passage is transparent for the developer, and that wasn't the case at all in old school frameworks.
Note that I'm not seeing this is good! but this is the value proposition of Astro and similar frameworks: transparent server-client context switching, with better performance perceived by the user.