I believe it, but it's not because of what framework they are using. React is only 100k. It's because of writing bloated websites without a concern for performance.
That includes resources like images, videos etc.
Javascript is 1/5 i.e. 420KB
The linked Medium article is over 3MB, almost 2MB of that being JS.
It's not the UI framework that's adding the bloat, it's a bunch of tracking and fingerprinting scripts. They would still be there even if the UI was server-rendered HTML, because that's Medium's business model!