I think it means that any page that only renders server components can just push the built html to the browser. If there were to be client components, like an upvote button, react would be required for that to hydrate the DOM.
It’s very similar to what Astro is doing. Only adding the JS if it’s actually necessary.
This video actually does a pretty good job of explaining the state of web today. He goes into why frameworks that stream html as updates are insufficient. The GitHub UI for example does this and its full of inconsistencies. https://youtube.com/watch?v=860d8usGC0o&feature=share