I don't see how you could interpret my comment that way.

A junior dev might use a single language because they apply what they know and don't go outside their comfort zone. A senior dev might use a single language for every other reason.

Everyone has a language they like best and spend a lot of time in (due to current job or side projects or whatever), but a developer who has never branched out to other languages and paradigms even to experiment for a bit (maybe bring back some knowledge to their preferred language) is someone I wouldn't want to work with.

But this is all from my perspective in a small startup where people have very broad responsibilities. I can completely understand a very senior developer working in a specific field (usually it's the low level stuff with C etc) who only works in one language because there's just no opportunity to use anything else.

Or perhaps someone who just sees their job as a 9-5 thing and looks elsewhere in their life for learning and growth.