A couple of the themes of Overland are Paranoia and Survival. You may not want to work with a demon cultist, but can you put aside your pride and drive to justice when a pyre city can be destroyed and the cultist is the only one helping you?
Now, my Overland posts and material for players will put down common knowledge of characters in the pyre cities. On occasion, to hide a later plot point, I will deceive or blatantly lie to hide a spoiler. DM Material will have what's really going on. I will not warn anyone, though I may put up a SPOILERS post for the DMs.
So, if you're reading Overland material and thinking that some of the information contradicts other information or one of my posts, you may actually be right. You might want to keep it to yourself anyway. You don't want to be exiled for inciting chaos now, would you?