Sede vacante is a dogmatic fact, which follows from the consideration of and assent to the dogmatic decrees of Cantate Domino, by Pope Eugene IV (Council of Florence), and supported by cuм Ex Apostolatus Officio, by Pope Paul IV, and numerous writings of saints and theologians.
And Antipope Augustine has proven himself to be a liar. He claimed to believe he was the pope, and then he did an about face and admitted he was a Protestant.
He is very dangerous.