From Issuepedia

A counterfactualist is someone who subscribes to a belief system that is counterfactual. This is most commonly because they belong to a belief-tribe which supports that belief system, though it can also be due to insanity. (It should be noted that beliefs espoused by belief-tribe members often closely resemble insanity, at least on the surface, due to their large degree of decoupling from observable reality.)

Counterfactualists can be said to practice counterfactualism.