Well he has two different arguments.
First he says they are null and void because they were signed by autopen. This is not true, you can sign in any way you want, including with a stamp or with your finger on a smartphone. Your signature is still completely valid and binding.
His second argument is that Biden knew nothing about the documents and someone else used his autopen to sign them. If this is true, Trump is correct, those documents are obviously void and the people who did that are guilty of several major crimes.
For the first one, is there something in the constitution that says autopen or other is valid? This is new technology which would require a constitutional amendment, no? What evidence is there that autopen or other is valid? I don't know about what the constitution allows, but I do know e-signatures are generally considered valid, but unsure about things of such magnitude as presidential pardons.
Then if he did sign himself the issue of him being senile would be different