Yeah, I don't think it's weird to reply to, "Hey, do you want to go out to dinner, as a date?" with "I'm not interested in dating, but I'd be happy to get dinner together as friends". I learned about most of my favorite restaurants in my current hometown by going out to dinner with friends who'd lived here longer (a married couple).

If the context was, "Hey, I read about this romantic high-end restaurant, would you let me treat you to dinner there?" and the reply was "I'm not interested in romance but I would like you to buy me dinner at an expensive restaurant", then that's more unfair/annoying/odd, but going out to dinner as friends doesn't seem strange in itself.