Before bitointalk.org creation, the forum was hosted on forum.bitcoin.org
Read the old post made by theymos, for more information.
The forum was originally located at bitcoin.org/smf, then forum.bitcoin.org (hence the redirect), and finally bitcointalk.org. Fun fact: The name "bitcointalk" was invented by Jeff Garzik, and he's the one who bought the domain.
It was mentioned on the thread you link it is sourceforge.net[1], although it's not a domain, it's like you can create a forum using the sourceforge.net website, which is the very first bitcoin forum.
As stated by Satoshi in that topic, The sourceforge.net domain was used before creation of new forum.
Satoshi created a forum on forum.bitcoin.org and that's now hosted on bitcointalk.org