In the second picture, the red marked areas indicate dates with big gap between the 3rd and 4th post in that thread. That is between sirius and PrintCoins posts. They are 1 year and 9 months gap between them. Does anyone also know why? Does it mean no other person posted between them until then or there were posts in between them but they got deleted? Was the forum not that active then?
I was not here back then, unfortunately, and I don't know whether theymos has done anything in there (removing posts, changes), but I can speculate.
By looking at ninjastic.space I've seen that "post #3" is the
200th post in the forum and "post #4"
is 665348th, so I'll say that people simply didn't care much of the welcome message and just posted in more interesting topics.
Even more, "post #4" is asking about the old forum, hence it's clearly not from the first wave of "welcome aboard" type of posts.
All in all, probably only theymos can answer this; and it's not something really important, I guess... You can pretty much live with "people didn't care to answer to the welcome topic".