A pardon is not "legally equivalent" to a guilty plea.  In fact, as established in Ex parte Garland, 71 U.S. 4 Wall. 333 333 (1866):

"A pardon reaches both the punishment prescribed for the offence and the guilt of the offender, and when the pardon is full, it releases the punishment and blots out of existence the guilt, so that, in the eye of the law, the offender is as innocent as if he had never committed the offence. If granted before conviction, it prevents any of the penalties and disabilities consequent upon conviction from attaching; if granted after conviction, it removes the penalties and disabilities and restores him to all his civil rights; it makes Page 71 U. S. 381 him, as it were, a new man, and gives him a new credit and capacity."