The reason it's a sign the investor isn't very smart is that there's no real distinction. There are a lot of impressive startups (e.g. Twitter) that could have been called mere features when they started. So all anyone accusing you of being a feature is really saying is that you're small, which you both already know.
But, if you can explain how this feature is just a trojan horse, and you're going to go across social networks, or introduce a dating service, or implement some sort of real-world trust/recommendation system, somehow building long term value and competitive advantage; then you look more like a business.
They're just asking how you plan to get there, or just to help them understand ways that you could get there.