I’ve never heard anyone claim to be allergic to artificial sweeteners, but I know people who claim to be allergic to things they don’t like. Why not? People don’t seem to respect their preferences until they pull out the big guns so they head off the argument by starting there.
I’ve claimed to be allergic to things that I don’t strictly have an allergy to, things that inevitably upset my stomach and cause… erm, gastric distress. It’s not usually something I want to be detailed about, so “allergy” is a nice way to just put that conversation behind us.