In json the code has to be escaped into strings, and json is really finicky about syntax (e.g. no trailing commas). So it doesn't work well.
I never got the chance to redo it, however the solution I was leaning to for my post "I won the lottery, I can work on fun stuff" attempt was to store the meta-code in a version of the host language(s), with some simple syntax that could live comfortably in the comments of various different languages to do things like encode the cell divisions and so on.
Basically something like: #notebook[lang=python]
#cell[lang=python] def add(x,y): return x + y #endcell
//notebook[lang=scala]
//cell[lang=python] def add(x: Int, y: Int) = x + y //endcell
This I think would be beneficial for a couple of reasons.
1. Better diffing / merging.
2. One click toggle between show source and view as notebook mode, which would really allow this to work in an IDE like vscode pretty seamlessly. The cells become something akin to //#regions in the IDE. But at the end of the day you are still editing a source code file, so you can edit the whole file easily.
3. The keyboard shortcuts for executing and jumping between cells would generally work in raw code mode, so you could just edit there continuously and manually writing out //cell //endcell. Also the executing results could appear in block comments inline in the editor, off to the side, or in a popup above, the code you are editing.
4. The IDEs could uprender the comment-syntax into cells as they gained better support for the paradigm (similar to how they do for code folding / syntax higlighting already).
5. Eventually, perhaps a cross language, metasyntax could be established to make things a bit more concrete than magic comments (get ready for some serious bikeshed painting though!)
The closest I have seen anything come in this regard is Quokka however it's not quite all the way there.