The Brookings report, presented in The Atlantic, ostensibly focused on how well a transit system connected people to their jobs. So they used "coverage", defined as how many residents are within 3/4 mile of a transit stop, as a major criteria.
"But residential coverage, as a primary indicator of transit quality, is a very loaded way of thinking about "best," especially if you care about transit sustainability outcomes that depend on ridership. Many people are within walking distance of a bus stop but not within walking distance of service that's remotely attractive in terms of frequency, speed and reliability."
"Due to this definition, the Brookings Atlantic list comes to focus on rather low-density, car-dependent cities that happen to have good transit coverage."