People who order in store will have a code and redemption instructions printed on their receipt, which I assume will mean that they can redeem the code on September 10.
However, what I believe IGN was referring to in relation to August 9 in their original article, is that starting that day, if you preorder you will have the code actually printed on your receipt, while if you preordered before you will not and will need to go into the store to get your beat token/code + redemption instructions.
Also, after checking GameStop it appears that the beta will run for 2 weeks (14 days), ending on the 27th of September.
You are right. Sorry about that after reading the GameStop website, I can see where you are coming from. It appears that if you preordered online before seats run out you will receive a seat in the beta, with your code and redemption instructions emailed to you on September 10.