My argument was badly put.

What is the benefit of using the same language on the client and server? That the developer only needs to know one language?

The server-side code you write has to interact both with the client and the underlying OS. So you should consider how language X interacts with both client and OS, in addition to the intrinsic qualities of the language.

If we adopt a modular approach to software development, then I don't see any universal benefit to using the same language for all modules.

js might be a good server-side language, but choosing it because it works on the client is the wrong reason.