Take a look at the map. Most of what’s covered is exclusively or predominantly single family housing today. In SF the houses are much closer to each other than in postwar suburbia, but all along our transit corridors, it’s mostly houses. The only neighborhoods this avoid are those which don’t even have bus service.
Upzoning is needed and transit is the right place to start, but we have to acknowledge that this would certainly have a massive effect on many single family neighborhoods.
I wouldn't say "many" single family neighborhoods, it's really only San Francisco, a city. Even where there are pockets of single family houses, it's not what people usually think of as "single family home neighborhoods." Dual-unit buildings are extremely common, especially in the avenues. And in the Avenues there are plenty of tall apartment buildings sprinkled throughout the neighborhoods, they just had to be built long ago because they are illegal to build today.
Upzoning is needed and transit is the right place to start, but we have to acknowledge that this would certainly have a massive effect on many single family neighborhoods.