That rule is specific to products labeled "Vodka".

Flavored vodka has to include the flavor on the label. Pages 2 and 11:

https://www.ttb.gov/spirits/bam/chapter4.pdf

Ah. Well, what can I say? I can easily tell the difference between different vodkas in blind tastings, though I can't necessarily tell you which is which. But my wife can consistently pick her favorite vodka out of a blind lineup.