They are referring to some fringe "tax protester" conspiracy theories which dispute that the 16th amendment was properly ratified. You can read about them here: https://en.wikipedia.org/wiki/Tax_protester_Sixteenth_Amendment_arguments#Sixteenth_Amendment_ratification
Suffice it to say, these 'theories' have been largely rejected, including by the states themselves, and by the SCOTUS.
They are referring to some fringe "tax protester" conspiracy theories which dispute that the 16th amendment was properly ratified. You can read about them here: https://en.wikipedia.org/wiki/Tax_protester_Sixteenth_Amendment_arguments#Sixteenth_Amendment_ratification
Suffice it to say, these 'theories' have been largely rejected, including by the states themselves, and by the SCOTUS.