The same sort of statistics that support marijuana being a "gateway drug" also support the same designation for alcohol and tobacco. (Don't know how anyone would arrive at exactly the conclusion you cite, though.)
Very simple: The guy just had to think, "I don't like tobacco, alcohol, or caffeine, but I like my pot. I think I'll write and article saying what I don't like is a gateway drug and what I do like isn't."
Me, I don't like tobacco or pot and I drink only occasionally, but if they want my caffeine, they'll have to pry the cola out of my dead, cold, frosty, refreshing hands.