My goal with this piece is to show that basic income is achievable while changing as little as possible of other things. Taxation would be included here, so the constitutionality of income taxes is orthogonal to my arguments.
That said, I’m curious how you would suggest financing basic income without income taxes. Personally I’d like to see more land value taxes and carbon taxes in our portfolio of taxes, which could lead to reduced income taxes. I also agree with most economists who believe the corporate income tax should be abolished, though personal income taxes are a very large part of government revenue, and so are highly unlikely to go away for several decades.