I interpret this saying a bit more like: quitters are those who stop trying altogether, and when you stop trying, you never win anything. Winners on the other hand, aren't always doing the same thing over and over again and expecting different results (they call that insanity, right?), but they're constantly tweaking and modifying, learning from what didn't work and they get back out there and keep trying...and eventually they get a "win". The final product often looks nothing like the first attempt.
So with that in mind, your point is totally valid, that winners know when to "call it quits" on a bad idea and then move on to trying something different.
As for me, I try not to get too hung up on a particular outcome or belief, because I recognize there are too many variables in life outside of my control. I have goals I work towards, but I always keep an open and growth-minded outlook to stay adaptable to constantly changing circumstances (both internally and externally). I wouldn't say I'm a "winner" though... I've definitely quit a lot of things, but I've also stuck with a lot of things as well. When I do make commitments, I take them very seriously.