It’s implied.
> I drink diet sodas. I have received many emails/calls/texts from friends and family telling me about the "news" of them being a possible carcinogen. To every such person I say: Alcohol is a KNOWN carcinogen. Drinking alcohol causes cancer. So don't lecture me about drinking diet coke until you have given up on beer
The poster compares the risk of aspartame being a « possible carcinogen » to Alcohol, which is a « KNOWN carcinogen », specifically comparing the different classifications.