We can see Satoshi's posts, but were they actually made here? Or were those posts imported from elsewhere like SourceForge?
As far as I know, Satoshi's posts were imported from SourceForge, which also includes the first four threads of some tests and some administrative notes. And since Satoshi was last seen active on December 13, 2010, it is clear that he never made a single post when Bitcointalk.org was launched in July 2011.
Topic 3 was deleted years ago, and it is unknown what happened to it. Topics 1 and 2 were satoshi doing some tests, and topic 4 was some administrative notes left by satoshi.
See below for PDFs of the first topics:
topic 1topic 2topic 4As an FYI, there are links to the above topics in the link you referenced.
Is this the first bitcointalk.org topic?
What happened with topic 1,2,3 and 4?
1, 2, and 4 exist in the staff forum. 3 was permanently deleted at some point (it must have contained Satoshi's real name and address).