You are correct that the whisky marketers don’t understand why GMO is a “a thing” and make an incorrect assumption, but you also don’t understand and make the same incorrect assumption.
Many people don’t care about GMO for their own health reasons (they trust the EPA, FDA, etc. — well, they did, until this presidency at least), it’s more for political reasons, i.e. on principle. Genetic diversity is a defense mechanism against bacteria, insects and even climate. A monoculture could be completely wiped out by one strain of clever organisms. Also, the huge companies selling monocultures care only about profit, and try to “lock in” their customers, arguably unethically. GMO increases yields which drop prices so farmers feel economic pressure to use GMO, then to use the special herbicides optimized for their strains, etc. Non-GMO and Organic labelling allows independent farmers to compete, by creating a premium product. Consumers can now “vote with their feet”.