The goal here isn't to convince them, but to get them to go away. The message is "If you want to convince me to drop diet coke, you have to be prepared to discuss your alcohol consumption."

It's not an academic conversation, nor is it meant to be one.

And yet it's the single most common rhetorical tactic I see employed by people to defend their own beliefs, especially political ones.

Seems to work on some subset of the population.

I works great for defense, terribly for convincing.

I don't think this is whataboutism, really. Part of it is frustration, but more it's an attempt to get them to recalibrate their idea of "carcinogen". Carcinogens are not some list of ultra dangers to strictly avoid. We're too good at detecting them.