in the past tense, which can only mean that the regime has fallen

Plusuntrue. This may simply be a conventional way of indicating that Orwell is discussing a fictional universe; there is nothing in the text to indicate that the narrator of the appendix resides within the world of 1984. Similarly, when SM Stirling ended "Marching Through Georgia" with an appendix of Draka history and weapons, it was in the past tense, but it wasn't obviously an in-universe work; it had information not readily apparent to any Alliance writer, but attitudes neutrally non-Draka.