There is nothing about these builds that is of, from, or needs Android L. The New Recents style was being worked on before Google I/O, but yes, was ultimately very similar in style.
Instead of nightlies like some ROMs, PA builds weekly (except this week of course) and tags the releases as one of Alpha, Beta, or Stable. Usually (and in the case of the New Recents testing period) when there is an Alpha, there is a normal (whether that is Beta or Stable) build released as well for people who aren't interested in testing new features.
Download Beta 2, it's very solid for the N7.
Thank you. Just to make sure I have understood correctly, I am gonna try to wrap it up;
1) There used to be 2 different versions a while ago, one was a stable one and the other was the one with very young new recents feature.
2) Now, since the new feature is matured enough, there is only betas which are the stable ones with new-ish but still beta recents option?