It's true on many levels, not the least of which is biology. You've heard of inbreeding yes? That's the opposite of diversity and leads to weakness and sickness in animals. Same for humans.
But that's not what we're usually talking about when we hear the phrase used today is it? No, at it's best, "diversity is a strength" means taking advantage of various view points and experiences and in that sense, it certainly is a good thing. Unfortunately, the woke have perverted the notion and made it mean that every human endeavor MUST have perfectly proportional representation, which is of course nonsense.
Does that help?