I'm sure that their claim, whatever it might be, won't be based on an infringement of the 16th. Most likely it will be an equal protection claim. That would still be very dubious since the rules apply equally to residents of all states, and it's only the behavior of the states themselves that lead to different results.
I agree with you. I think that the legal arguments in the linked article are just stupid. But winning a suit in the long run isn’t the point;’just ask the jokers who brought the “emoluments” case.
You file your lawsuit, get a friendly judge to issue the temporary injunction, and delay the undesired result for as long as possible,
Or just gum up the workings of efficient governing