There are people who are very against the two-party system in the US so they always vote 3rd party.
They have created a system whereby there are two groups of political parties - two party and non-two-party.
Therefore they fit into a new two-party system - becoming two-party again
Does that mean those who don't believe in being non-two-party are the least two-party of all?
No, because there are more options outside of the two parties, just as how there are more options outside of the binary (some examples: agender, bigender, genderfluid, genderqueer, + some others beyond the first link I clicked on from google)
Being against a two-party political system just means out of the dozens of political parties in the US you do not fall into one of the major two. Being nonbinary just means that out of the multitude of genders, you don't fall into one of the major two.