I think a lot of technicians view any sort of utility like CleanMyMac as unneeded. There are apps like MacKeeper and others that are (or are at least borderline) malware, and sometimes people mix things up or paint with a broad brush.
Apple pulls system diagnosis info for a lot of AppleCare cases these days. Sending them a .zip of whatever it’s collecting isn’t super uncommon.
If your issue was just not beating able to install from Setapp, have you reached out to them, or am I misunderstanding the original reason for the support request?