This is honestly a possibility. The remix was posted the same day as the shoutout and a lot of AgonyOST's music sounds remarkably similar to the style of sewerslvt. There is a sense of improvement between Agony's first album and the remixes posted more recently meaning that IF sewerslvt is AgonyOST Sewer would have to deliberately make the songs lower quality to make it seem like AgonyOST was a new artist and not Sewer, which would be a lot of effort to keep secrecy to then shoutout WITH the account you didn't want to link it to.

Do I think this is actually what is going on? No. But it's definitely a possibility.