Very much this. There are many de facto one party districts in the U.S. where a third party wouldn't run into the issue of being a spoiler. The Vermont Progressive Party only runs candidates like this. The result is that thought they're only active in Vermont, they have 11 seats in the Vermont state legislature. In contrast, the Libertarian Party and the Green Party are across in the entire U.S., and out of all 50 state legislatures they have a combined total of 2 seats (2 for the Libertarians, 0 for the Greens).