It was a bad joke from theymos and most likely the topic was a test of the forum's delete feature as topics 1,2,4 were related to the forum test.
According to the same thread where theymos wrote what he did, threads 1, 2, and 4 still exist but they are accessible to the staff only, not to the general public. Thread #3 is, thus, different from those. There is always room for speculation. I don't think thread #3 was deleted just for testing purposes. It might have contained something that shouldn't have been there. Like I said, we will never know, but it's fun to speculate.