Nixon was likely going to be impeached, removed and then would have endured a federal prosecution for any number of crimes. His resignation eliminated the first two, but not the last one, which was removed by Ford's pardon.
The only way to prove Biden didn't authorize the use of his auto-pen to sign the pardons is for him to personally admit that he did not. Why would he ever do such a thing?
In the military, I had "by direction" authority of my commanding officer. It was a letter stating what and under which circumstances that I could assume my commanding officer's authority to sign for the command. Waivers for enlistment had to be done by the commanding officer himself in most cases. If he was unavailable, he could authorize me to sign in his stead. I had the same authorization if my executive officer was unavailable.
I doubt Biden signed a letter listing all the people he was pardoning and authorized use of the auto-pen to sign the repetitive paperwork of the pardons, but he very well could have.