I would imagine they have a list of the default / max DHCPv6 blocks that ISPs hand out. They are generally public knowledge so it would be easy for them to say Comcast hands out /56 so when blocking they block at that level for the ASN.
It probably makes more sense to derive that information from measurements. There are a lot of ISPs in the world, and allocation policies can change, even if you manage to find a person who knows what they are.
