The entire language in the Constitution pertaining to pardons is “he shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment”.
So a President can issue pardons only for “Offences against the United States”, yet the Biden pardon states “The issuance of these pardons should not be mistaken as an acknowledgment that any individual engaged in any wrongdoing, nor should acceptance be misconstrued as an admission of guilt for any offense.”
So, in the absence of any offense (according to Biden) there are no “Offences against the United States” for which pardons can be granted.
In the US, no one has committed a crime (”Offence”) unless he or she has been convicted in court, or pleads guilty. So, the President has no authority under the Constitution to grant pardons when no “Offence” has been committed.
Section 8 and Amendment 5 of the Constitution both imply that the meanings of “offence” and “crime” are the same. The Fifth Amendment discusses “crimes” and “offenses” in its provision related to double jeopardy.
And yes, if the SCOTUS were to rule that no such pardon power exists, it would render the pardon of Richard Nixon illegal and invalid. But since Nixon and Gerald Ford are both dead, the issues is moot.
The Supreme Court decided in 1915 that this is not true.