That's not exactly how the math works. It's just a dumbed down, oversimplified, and purely rhetorical catch statement to enforce the two-party system, or to say a vote AGAINST one major party candidate is AUTOMATICALLY FOR the other. The actual math, when it comes down to the actual counting is much more nuanced and, sometimes, quite surprising, in how it turns out. In 1948, two schisms off the Democratic Party running their own separate campaigns, tickets, and ballot nominations, and one of those even gaining EV's, and the other respectable vote tallies, was thought to hand Republican candidate Dewey victory for sure. In fact, the Chicago Tribune even printed their papers for the day after election night declaring Dewey's victory without waiting for the election results. Truman, the Democratic incumbent, won that election. Case-and-point against the rhetoric I'm quoting in your post.