Domestic and Flag air carriers have the airports they are approved to fly into listed in their Operations Specifications. 121.637 is for when, for some reason, they end up at an airport not listed in the OpsSpecs. There is normally no ceiling requirement in FAA land. However in this case there is. So if we were at an unlisted airport, in order to use the minimum visibility allowed per 91.175(c), we would also have to have a 1000 foot ceiling. If the ceiling was 800 feet we'd need 2 miles vis. If the ceiling was less than 800 feet, per 121.637 we're stuck there.
The ATP knowledge test covers the regulations as published for everyone. In the real world most airline's OpsSpecs get paragragh C078 which covers lower than standard TO minimums and, depending on training, airplane equipment, runway lights and markings, and the number of RVR sensors, can get you down to 300/300/300 (75m/75m/75m).