That is a complete non-sequitur. Json in no way enables that. Just having a defined format enables that.
Emacs org-mode is proof that a simple text format with markup rules is all you really need to support multiple languages in a single file. You lose some of the simplicity of parsing the file, but you gain a ton more.
This isn't true though, right? If you were writing an org mode document about org mode, you now need an escaping mechanism to not mix your structure and text
Multi-language parsing is much harder to solve than simply enforcing some escaping mechanism in the inner protocol level and having tools do the "heavy" lifting (basically a solved problem).
That said, you can define away a large part of the problem.
Edit: For trivial examples of "org-mode" in an org-mode document, you need only look at the documentation of org-mode. That said, I expect there to be limitations, because they make sense. Similar to how you can pretty print json inside a jupyter notebook, but don't expect to have a notebook interpreted in the notebook. (If that makes sense.)
Emacs is, amusingly, a lighter client than most browsers nowadays.
On point, a browser can not remember ifa notebook. Just parse the json. It can also parse text/plain. So, could show the org document without styling. The org document is actually readable. Json... Not so much.
To see the notebook, you have to have a Jupyter setup somewhere.
How is a notebook without proper software to handle it in any way more useful then other structured plaintext-file? Yes, JSON can be prettyprinted in a browser, but what then? It's still a useless mess you can't work with.
Emacs org-mode is proof that a simple text format with markup rules is all you really need to support multiple languages in a single file. You lose some of the simplicity of parsing the file, but you gain a ton more.