That's absolutely correct. The gov't purposely avoided using the term "winner", because even if you get a number it is no guarantee that you will get a visa/green card. The letter says you are "selected for further processing", and that the number of applicants selected is greater than the number of visas available. Technically you're not a winner until you have that visa in your passport and/or the green card. As such, you are free to apply for the next year DV.
There's somebody on this forum who has a high DV2013 number and he couldn't get the interview in time, so he's not getting a green card this year. Fortunately he entered and is also selected for DV2014 with a lower number, so he gets another chance at getting GC. I bet he'd enter DV2015 too just to be safe.
Even if you already have a green card there's nothing in the law that punishes you for entering the lottery. It's pointless, of course, but not illegal.
That's amazing. Who is this lucky person? I would pay to have their secrets of being this much lucky.