I think those acronyms are used to protect the board, not the poster. The boards that I know of where those sorts of terms are used do not want any ‘confessions’, essentially - I think it has to do with potentially getting shut down. This board has similar rules, if my crusty memory is to be believed. You can’t start a thread admitting you’ve done anything illegal or linking to illegal material.
On those MBs, the “SWIM” or “SWIY” is an attempt to get around that, I think.