MSM touting an untruth. That is not spelled out in the Constitution. If it comes down to who follows the VP the decision will be made by SCOTUS.
The Presidential Succession Act of 1947 (3 U.S.C. § 19) establishes the order of succession to the Presidency in the event neither a President nor Vice President is able to "discharge the powers and duties of the office."
The authority for Congress to do this is in Article II, section 1, clause 6 of the Constitution: "In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the same shall devolve on the Vice President, and the Congress may by Law provide for the Case of Removal, Death, Resignation or Inability, both of the President and Vice President, declaring what Officer shall then act as President, and such Officer shall act accordingly, until the Disability be removed, or a President shall be elected." (Modified somewhat by Amendments XX and XXV.)
The order of succession:
Speaker of the House
President Pro Tempore of the Senate
Secretary of State
Secretary of the Treasury
Secretary of Defense
Attorney General
Secretary of the Interior
Secretary of Agriculture
Secretary of Commerce
Secretary of Labor
Secretary of Health and Human Services
Secretary of Housing and Urban Development
Secretary of Transportation
Secretary of Energy
Secretary of Education
Secretary of Veterans Affairs
They didn't say it was in the Constitution, although the Constitution does give Congress the power to determine the order, which they did, in several instances, the latest being the Presidential Succession Act of 1947 codified as (3 U.S.C. § 19)
So unless someone challenged the existing law, the SCOTUS would not be a factor. In the circumstances, the death of both the President and Vice President at the same time, or of the President who had succeeded from VP when the former President died in office and before a new VP could be selected, it's highly unlikely that there will be time or inclination to run to the Court. The authority for Congress to enact such a law is Article II, section 1, clause 6 of the United States Constitution.
You need to reread the 20th Amendment to the Constitution. It clearly gives Congress the authority to make the Speaker of the House the successor to the Vice President. The Supreme Court has no say.
Actually, the Constitution says Congress by law is to designate which "officer", meaning "official", is to become President if the presidency and the vice presidency are vacant. It used to be the President Pro Tem of the Senate and now it is the Speaker of the House. No need or reason for the SCOTUS to get involved.
The Speaker of the House is 3rd, Constitutionally.