A pardon merely sets aside the risk of punishment, regardless of whether such legal punishment is correctly given or injustly given. Ask yourself: Would a pardon protect him from an unjust prosecution? Of course it would. Therefore, it is not an admission.
I know at least one justice has said it may be an admission of guilt in the context of discussing whether or not a pardon could be turned down, but that is not a matter of law; it is a matter of opinion, which means it is not, legally, an admission.
And, why would he not just pardon himself in secret?...and keep the pardon in a safe place in case he is convicted of a crime for which he is unwilling to pay the penalty? That would be the smart thing to do. But, you have a point: he may be too arrogant to pardon himself.