OK, to clarify, the problem is in the thread that's titled "Thanks for the mammaries", not the "Thanx for the mammaries! - Best Gag Photos of All Time" thread. (The similar names have been confusing the moderators for about a decade.)
I think what happened is that Salem (the Watchman who's in charge of merging redundant threads) found another forgotten old thread named "Thanks for the mammaries" and merged it with the same-named thread that gOOber started. Messages in merged threads are arranged in chronological order, so if the thread created by Sleeper was started before the thread created by gOOber, Sleeper would become the originating poster of the merged thread. (Incidentally, Sleeper hasn't posted since 2015.)
I can't get any of the links in Sleeper's post to work at all, so I'm not sure what to do about the warning message you received.