I believe when he went over that part in the video he was commenting to what cities sometimes do when a subway is built under a street, they rip out the tram lines at street level as a default.

I believe the comment was that if a subway is under a street it would serve less stops (greater distances between stops), where a tram at street level would serve the same stops as the subways but with two or more stops in between subway stations.

Toronto as a example has a subway with a bus line at street level. Though it only operates the bus line if the subway line is shutdown. With the two operating at the same time it would decrease some of the crowding at rush hour and allow the bus to serve the intersections between subway stops.

It seems redundant but its similar to needing to jump in a taxi after leaving a subway station to get to a stop in between stations.