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.
Careful how you phrase this. The right wing anti-woke crusaders will retort "oh, so you're saying this level of CO2 is natural after all?"