It could (conceivably) be that "in San Francisco" is meant literally, and that in San Francisco it's true, even if not the case for other instances of "Main Street" elsewhere, however; a cursory web search leads me to think "urban legend" is by far the more likely. It's always been my understanding that "Main Street" either is currently, or was at some time nearer the town / city's beginning the main street that went through "downtown".