Untrue. The 16th Amendment makes it constitutional: The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration..
(Passed by Congress July 2, 1909. Ratified February 3, 1913.)
Article 1, Section 9 states: No Capitation, or other direct, Tax shall be laid, unless in Proportion to the Census or enumeration herein before directed to be taken. I've underlined the part that is negated by the 16th Amendment.