Lots of things taste like vanilla, but that generic vanilla taste is as authentic as the generic banana taste. Real vanilla tastes very different from the chemical imitation that you find in every cheap dairy product. The "plain" in this story is refering to that fake vanilla. Real vanilla is still very much rare, expensive and exceptional.

Edit: fun fact, the fake banana taste is actually based off of the "gris michel" banana. This specific banana cultivar was THE generic banana you could buy in every grocery store. Until the 1950's when almost the whole species went extinct due to a decease. The "panama disease". Heavy monoculture farming made the banana industry extremely suseptible. Within a few years, all major farms were destroyed. Now we have a different banana cultuvar, the "Cavendish". This banana is not susceptible to the panama disease,but variations of the panama disease are allready showing up around the world.