There's no way this covers "all available carriers". Car insurance is a commodity product with a negative switching cost. The insurance companies know that making price shopping easy for consumers is shooting themselves in the foot. This website probably has a few companies participating at most, and is more likely to lose carriers than add
Most carriers benefit by being on the comparison site, while some still prefer only selling direct.
This is EXTREMELY similar to the OTA (Online Travel Agency) like Expedia, where most carriers benefit from being searched against many other carriers. Having said this, Southwest Airlines has never submitted to the OTA and is still only available directly. In theory, United Airlines loses out by being on Expedia, however, United obviously sees that they lose more by
not being on Expedia.
Back to insurance, most companies allow themselves to be compared against other companies.