Am I reading this correctly? They are advocating giving up on structuring code in separate files? Is that a scoping problem that NextJS has, which makes it difficult to use multiple files? Seems like a rather ridiculous statement of a framework to make.
It's the single file that is used to define the middleware function that runs. You can import whatever you want into it and decide how "you should combine [your middleware] into a single file."
As a JavaScript developer, combining/consolidating middlewares manually just defeats the design of middleware. I would rather call it a callback function.
I prefer this to having app.use / router.use scattered anywhere throughout the app init (i.e. Express)