But while everything is "possible," nothing is inevitable. Stalin's Russia created horrors that were similar to, though different from, the horrors created in Nazi Germany. The point is, after this history, we must now all realize that "everything" is within the realm of possibility. If we an imagine something, we can accomplish it (at least, it is not impossible for us to accomplish something that we have imagined). Maybe we should start having some "good" thoughts!
I think there is a good argument that the horrors perpetrated by totalitarian states are largely based on the acceptance by large parts of the society that there is, in fact, no "truth," or that the truth "doesn't really matter." Within Russia, a condition might exist that conforms to Pomerantsev's title: "Nothing is True and Everything is Possible." If such a place exists, then this is a warning that we must never concede that there is no "truth."
"Everything" really is possible, so if we can no longer test human action against the truth, the "everything" we will encounter (and create) will become a house of horrors, indeed.