Added in the early 1900’s.
NOT part of the original Constitution.
So? Constitution provides for an Amendment process. The 16th Amendment was ratified. I don’t like it. I don’t think it was a good Amendment to add, but it was, and it is part of the Constitution.
So your statement was categorically false.
By the way, if you want to be technical, none of the Bill of Rights were part of the “Original Constitution” either, that doesn’t make any of them less valid.