I think the J-mag comment is a reference to a story told by the mangaka of Attack on Titan about how Shonen Jump rejected his one shoot for the series by telling him it was a good manga but they were looking for Jump.
Usually S means out of the ordinary, so it could be a sarcastic categorizing of an absolute worst pit of publishing companies, not even worth writing in proper kanji for a company?
Thanks for the chapter, I think that the S is for two things, the first one for special tier, and the second one for sea, coinciding with the 海, with the possible meaning of a sea of sharks, or something like that.