As far as i know that should be the access for just beta 1.

Preordering today would grant you access from beta 2, that will begin next month.

I'm no official source, though.

Quoted from GPG email I have JUST (!!) received and read:

This is a limited time offer. Signups for this cycle of the beta test 
close on Friday, September 19th, though the beta itself is ongoing. 

The "offer" is the chance to get into the beta if you buy the game.
So until the 19th, if you get the game you go into phase1 of the beta.
Elsewise, you'll have to wait until the second phase.

By "close" it is meant that we will return to queueing new preorders until the next phase. If you already have the beta, you will have access to subsequent betas and can continue playing right up until release.

On that note, we haven't closed beta 1 just yet (Sept 11 was the earliest we would have done so). So you can still preorder now and get into beta 1 if you have not yet.