The comment doesn't make any point about relative risk. They're only mocking their friend for telling them about something unhealthy when their friend also does unhealthy things. As if it somehow cancels out.
> 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.