It has also been said elsewhere, that oxygen-based and CO2-based life are actually relatively rare in the galaxy, and that methane-based life is in fact more common. Is this true? And if it’s not true, why perpetuate such a lie?

This is not true. This is a deception and is meant to disguise the fact that there are great similarities among intelligent life forms throughout the galaxy. It is part of the cover story they maintain to keep the fiction alive that life is rare and the likelihood of encountering an extraterrestrial civilization is practically nil because there is no evidence that life exists beyond the Earth itself and may be a singular accident to begin with, and with the great interstellar distances practical travel from other planetary systems of distant stars would seem to be impossible based on human technological capabilities and what they can imagine doing with no means of propulsion. All of this of course is scientific ignorance because there are multiple life forms capable of space travel at this point far ahead of humanity in technological prowess. The Dark Extraterrestrial Alliance controlling the Earth has wanted to keep humanity in the dark all through its subjugation and suppression almost from the very beginning of the appearance of humanity on the Earth. Stories of this kind serve the Extraterrestrial Alliance to make life on other worlds seem improbable and that even if such life forms existed, the chemistry might be so exotic in their atmosphere and biological makeup that their worlds would be incompatible with yours, so any kind of interaction would be extremely unlikely and might be impossible because of basic incompatibilities. So this again is a cover story to keep people in the dark about the diversity of life forms that exist throughout the universe and not just your Milky Way Galaxy alone.