consider something small like when do we have the block party and two dates are getting most of the votes and either is likely
And this is where you actually disprove the two party system.
Obviously if you have a regular style vote (first past the post) and two choices are highly favored then you will have a two party system. But as you just pointed out it requires the axiom that you have 2 highly favored choices (not 10, not 1).
So really all you are saying is that when there is a 2 party system (2 parties are very likely to win) then you have a 2 party system…. Says nothing about us inherently having a 2 party system.
in fact the fact that we have seent he 2 favored parties change a total of 8 times in US history while the current voting system remained inplace disproves the idea of a two-party system