I agree with you in principle, but I seem to recall HomerunHall saying that anything >= 3 points overgraded (presumably 3 intervals) would be a mechanical error when he disclaimed the MS70 Ike that was slated for auction at Stacks. Of course that doesn't make HRH right either; it merely means that PCGS won't honor its guarantee.
PCGS is flat out wrong on this coin, but I disagree that the seller is not at all at fault here. He/she knows how to somewhat accurately grade a Walker and can identify a cleaned coin. The majority of this person's listings seem to be numismatic items. He/she should know better.