Not exactly. They are part of the mainstream, but they are not the mainstream.
A teacher is part of the school system but the teacher isn't the school system. Mainstream is synonymous with "mass media" as you've been told already. If the story isn't known by the average news follower nationally, it's not been covered by the mainstream media. If i piss in the main water supply everyone drinks piss. It's not like the water I piss in only goes to a few people. It's the main stream, but when you populate it with something, it goes everywhere, not just to the local community.
Is CNN the mainstream media as well? Did they report on it? Therefore using your method above, the mainstream media did ignore the story.