IMO SGC slabbed it as a "card". even if it says "stamp" on the label. SGC is not a "stamp" grading company. They are a "card" grading company. So it's a card in the spirit of what the authentication program is about, regardless of what anybody's else's opinion is on the matter.
This is a low stakes debate however. As long as it eventually gets to the buyer in safe order, then all is well with the world.
