This is just a misapplication of the analogy. For a language, "fitness" refers to similarity to whatever language is spoken by people relevant to you. Diversity is the worst quality a language can exhibit, and is the quality that causes dying languages to die.
There is no such concept as an external force coming in that certain languages handle better, allowing them to temporarily outcompete other languages. Existing pools of diversity are not protective against this, because it can't happen.
Also unlike genetic diversity, linguistic diversity does not need to be maintained as a legacy of the past. It is constantly being generated in much larger quantities than are desired. If you managed to perform the opposite of the Tower of Babel miracle and replaced every currently-spoken language everywhere in the world with a perfect monoculture, within 1-2 generations you'd be back to having mutually unintelligible varieties in different regions.