It seems that either the question is misreported or something is missing from the context. If the question were asking whether ‘they’ have proved that humans had existed for MORE than 6000 years, then it was almost certainly a religious question. Though if it is about a discussion with JWs, then it’s almost certainly a religious question (with only incidental relevance to science) anyway. Perhaps the question is meant to say “only” instead of “not”?
I’m not aware of any statistically significant view suggesting humans have existed for less than 6000 years (though there are minor and sometimes purely conjectural views that everything could have been made ‘as is’ only a very short time ago), and it’s unlikely that a JW would be discussing those views.
But the other aspect is that if the person specifically asked for information about proving a view false, the response that “It doesn’t matter” isn’t particularly helpful and might understandably elicit some irritation. Perhaps “I don’t know” would have been a better response if you couldn’t (or didn’t want to) provide any helpful information.