Isn't UW just an engine, not a game?
No, it is confirmed by Epic that UW is an actual game in the making by Epic.
It was first confirmed by Epic's Vice President Mark Rein himself in this interview on FGN Online, published Jan 22, 2001:
? Epic VP Mark Rein, who explained, "Unreal Warfare is the title of our next game and one of our worst kept secrets. Aside from the title, we're not ready to say anything else about the project at this point."
Furthermore Cliff "CliffyB" Bleszinski confirmed it in this interview on XGR (part IV):
You know, I'm not gonna lie and say we're not working on something. Yes, we are working on a game. Yes, the name is Unreal Warfare.
He says a few more interesting things about UW, U2 and MMOFPS games in this interview, particularly in part IV. If you haven't yet, listen to it.
Last but not least, Cliffy said in this interview on GamingGroove:
I can't really say much about what I'm working on. Rest assured, it is a game. And odds are, it'll probably have guns.
I'm quoting that because of the "guns" part. I knew you'd all love to hear that UW isn't a pinball game.