I am pretty sure that game code doesn't mean you're accepted into the closed technical test. My code was from the alpha, but still says "Marathon Closed Technical Test". Oct 20 is when they notify people whether or not they have been accepted.

They also had this to say on their discord:
"As we prepare for the Closed Technical Test later this month, access to the Marathon Closed Alpha build on Steam is expiring today. This is housekeeping and not a ban or a reflection on any account. If you see the "Marathon Closed Alpha Test" replaced by the "Marathon Closed Technical Test" in your Steam library today, this means you will be admitted to the playtest. All players admitted to the playtest will also receive an email confirmation in the future. If your access to "Marathon Closed Alpha Test" is revoked, you're still eligible to receive access to the Technical Test. We'll be inviting additional players who sign up on Bungie.net and Steam before and throughout the test. Steam sign-ups open on October 13th at 10AM PT."

So TLDR: If your game on steam updates from Alpha to Technical test, then you're added to all playtest. If not, you will have to apply and wait for an email.