Satoshi created Bitcoin, then founded Bitcointalk to discuss it. It's really not rocket science.
Just to rephrase Satoshi registered the domain bitcointalk.org but he did not create the forum, this forum is from
http://www.simplemachines.org/as it says in the footer
Powered by SMF 1.1.19 | SMF © 2006-2009, Simple MachinesIt's possible some customization must have been done by him but still the original forum was not created by Satoshi.
Wait, what are you talking about? Nobody has ever said he created the Software that the forums use. But Satoshi did create this forum as you can see in his
welcome message. SMF is a well know SOFTWARE to use to build forum communities.
Before creating bitcointalk, he used sourceforge board and mail List.