“The 20th is worded thusly: The terms of the President and Vice President shall end at noon on the 20th day of January, and the terms of Senators and Representatives at noon on the 3d day of January, of the years in which such terms would have ended if this article had not been ratified; and the terms of their successors shall then begin.”
What part of “shall end at noon” and “shall then begin” is hard for you? It comes after. It supercedes. The End.
Yes his TERM shall begin However Before he enter the Execution of his Office he shall take the Oath.
IOW his term starts at Noon and will End Four Years later at Noon but he can't Execute his Office (Sign Legislation and Treaties ETC.) till he takes the Oath.
Its why he retook the Oath and its why Constitutional Scholars said he must retake the Oath.
QED