It's an argument that's so beloved by professors, but there was not a single justice who was even interested in the tax-power argument in six hours of oral presentation. If any justice liked the tax-power theory, you would have expected to hear from her or him during the discussion of the Anti-Injunction Act.
The justices were debating among themselves why it didn't apply. There was nobody saying it did apply.
If it were actually accepted by the Court, they would basically be saying that Congress has unlimited power to do whatever they want as long as they limit these sanctions to a monestary fine collected by the IRS. That would be a startling claim of power.