This has nothing to do with multithreading, though?

Absolutely correct. Those are runtime execution context-issues. There are other frameworks that do force you do deal with those (and in other languages, probably), but I believe in next.js the difficulies are at another level - because of poor documentation and the built in abstractions to allow for running next.js both in development, node.js server, and edge.

Not OP but wasn't the article about lots of async/await context issues?

If the handle() method of the middleware API would have provided, say, a context.Context parameter, most of the described debugging issues would have been gone, no?