No, they do not have to admit guilt to get a pardon. Often a pardon is granted because the accused was actually innocent (Not Marc Rich). But there is some authority which says a pardon must be accepted to be effective.
That came up in the context of Trump should just pardon Hillary for her various crimes “for the good of the country”, but then the witch would tell him to shove it and not “accept” it.
Thanks for the info. Rich even were he pardoned would not avoid the tax evasion.