Unlikely, but possible. Large web hosts end up on the list from time to time, and a lot of innocent sites are blocked in the process. It's an incredibly difficult process to find out what site triggered the initial block and to get your IP's removed from the list.
Yes, they use IP's. They don't do URL-level filtering. They use IP's.
That system has blocked wikipedia before. Fortunately people made a fuss and that block was quickly removed, but accidental or deliberate censorship of a less well known site could easily go unnoticed for months or years.