Probably they're just a reseller, like Tesco or JLP.
Certainly those two are not listed, and certainly I think this site should try to list all ISPs as otherwise you can't trust comparison lists.
Interestingly, the government appears to want to stop the likes of moneysupermarket.com from having contracts with energy suppliers which stop the suppliers from selling at a lower price elsewhere. The effect of this is that there is no competition between the comparison web sites which the government think is wrong.
I would prefer to be able to trust the comparison sites!