It is not testable
Setting up an app, like Express' readme states doesn't need to be. I'm confident that they test it.
Any dev inheriting a project should not be making assumptions
You are correct. But you also shouldn't be wrapping code in unnecessary promises.
If you think wrapping it in a promise harms maintainability
In this case, it does. A class for handling 3 lines of code is so unnecessary. The lines of .then() makes it more difficult to read. I'd actually suggest you took a class in writing maintainable code. I'm happy with the quality of my code, and my employers agree.
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If you've set up express, you'll have used my example. Not some over-engineered promise wrapper.
If you're unfamiliar with express, their readme gives a very similar example, at the top. github.com/expressjs/express/blob/...