What will really blow your mind is that satoshi himself never posted on bitcointalk at all.  It was hosted on a different domain back then and it wasn't until satoshi's "disappearance" that bitcointalk came to be.  As for your question, satoshi wasn't the admin of the old forum and joined after it was up and running.  If you use the search feature you'll get a lot more detailed knowledge about exactly who started the forum and the timing involved.  In the meta section this has been discussed quite a bit over the years so you should be able to find a few detailed topics fairly easily if you search there.