I don't see how honesty has anything to do with it, unless there's some rule I don't know of whereby being honest precludes someone from making a mistake, but that's clearly what happened. Is it more likely for an honest person to have real cards and believe them to be fake (I've done this myself at least once) or for someone to have invested tens or hundreds of thousands of dollars to create convincing fakes using appropriate images whose appropriateness would not have been known until several years into the future; and, having already demonstrated himself capable of reproducing the printing and cardstock that would have existed in real cards, to modify them sufficiently to convince two people that they weren't real, all the while having no intention of doing any of this for a profit but being motivated instead by the fun of investing years of income on a lark that quite likely no one else in the world would ever have found out about? Honest (and self-injurious) mistake or time-traveling spendthrift? You decide.
But good luck persuading the guy who missed the chance to pick them up for a few bucks.