I felt like I might come off a little disingenuous telling you I wanted the frame. I am in fact looking for a 58-59cm frame. The one in the picture turned out to be super rusty and had damaged chainstays, as I said before.
A bike's weight is not supposed to affect performance that much. It may lead to a race-winning edge but in casual use it's no big deal.
The reason frames made from thin wall, high quality tubing are sought after by non-racers is the fact that they are supposed to have an excellent ride. If the bike really weighs 23lb it definitely belongs in that category. I suspect, though, that it may be a bit heavier than you think. Since much of the weight comes from other components, a difference of a few pounds can really mean a huge difference in the thickness of the tubing walls.