That is a vast oversimplification of how Twitter works. People absolutely see tweets from people they don't follow, all the time. Sometimes it's a promoted tweet, sometimes a retweet, sometimes just by browsing through trending topics. It's pretty much impossible to use the platform in such a way that you'll only see tweets from people you follow, and nothing else.

Agreed that Twitter isn't a message board, but that's a much closer analogy than calling it a phone call.