Is it? I had to write a bunch of µBlock and usercss rules to make their articles somewhat readable. The sticky bars waste a lot of screen estate. Firefox reader mode (Ctrl+Alt+R) is a wonderful thing, but it breaks on some pages (I guess due to Medium's incompetence or brain-dead priorities since they're still using gist for code snippets (every single snippet in a separate iframe!) instead of spending half an hour to integrate highlight.js)
I'm not sure what it does really but you can achieve a similar design with a few Kb of CSS.
They all load almost instantly.
Is it heavy for such a simple blog? Yes absolutely. Is it heavy for a modern React app? Not really. I think the crux of it is that we often rely too much on heavy frameworks for React even for simple things. It's definitely an antipattern in development today.
Server Side Rendering(react or w/o react) would be a better way to do serve their content and have the added benefit support users who have no js support (noScript , terminal based browsers, screen readers? etc)
(Given your numbers and the ones in the talk, it would appear medium pages have doubled in size since the talk was given).
But yeah, I think folks reach for heavyweight frameworks too quickly. You shouldn't be using React for a blog.
I would think that 0.5mb should be more than enough.