Their statement was appropriately hedged. They didn't claim small amounts were harmless, just that it wouldn't hurt in a "serious" way. Your rush to "um actually" someone meant you missed what was being said and "um actually"'d a statement that existed only in your mind. Anyway...

Everyone's threshold for "serious" harm is different.

For adult men, if you drink fewer than 15 drinks per week but greater than zero, the expected decrease of average lifespan is measured in months not years. Obviously more than zero. Is that "serious" enough to not partake? Up to each person.