I think he is not in the Yolo signature campaign, he just put Yolo signature with his referral links.
I am one of the Yolodice Signature Campaign participants, YOLODice provides a unique signature that contains referral codes for each user's account in their platform 
. So basically, all of the campaign participant wears a signature that has referral code attached given from their Yolodice account that can be found on "Referral" section.