The typing actions have to be observed by JavaScript. It's not different to any other JS blocking page load because it's needed for the site to work, that's just how the web works.
(Separately, I don't think the article has adequately demonstrated this claim. They just make the claim in the title. The actual article only shows that some network request is made, and that the request happens after the React app is loaded, but not that they prevent input until it returns. Maybe it's obvious from using it, but they didn't demonstrate it.)