They need to get the store up and running, the download will probably be accessible through some part of the store (as will future alpha and beta releases, and eventually the full game) - One system to manage all the releases makes good sense (I'm guessing here) - Yes, I know purchasing access to Alpha will NOT be available from the store.
Given that, the earliest I would expect the first Alpha release to be available on or around Friday 13th December, unless FD are superstitious. This gives enough time to get the store up and running, iron out issues and test the possible distribution systems.
All guesses, based on my own experience of releasing stuff (I am a software developer)