Even if he could, Musk would have to accept the pardon which involves admitting his guilt.

A pardon means punishment is waived. You have to be punished before a pardon applies, and you have to be found guilty before you can be punished. So the only way to avoid the trial is to plead guilty/no contest.

A pardon is not immunity, immunity means you cannot be tried so the question of guilt is forever left unanswered.