Indeed, as there is no contention that this was addressed to believers, but who can fall away, (Gal. 5:1-4; Heb. 3:12; 10:38) and only location condition of any unfaithful servant is with the lost, with the latter two dealing with the degree of punishment. It is up to you to prove that this refers to purgatory. Just accept that this text does not teach that.
Neither is the word believers in the text. This is about servants, friends, and a flock. It is up to me to believe His teaching and obey it. The teaching clearly admits different levels of reward and punishment among His servants and friends. The Apostle Peter proceeds to admit us into the room, as it were, with his question. Proof texts are not the issue; after all these centuries there should be no divisions if proof texts were the solution. Obedience is the issue; this is something each one of us must do; may we have a perpetual fear and love of His Holy Name.