For example if the FAI scenario is much less likely (a priori) than a Clippy scenario, then there's no reason for Clippy to make strong concessions.

But if a "paperclips" maximizer, as opposed to "tables", "cars", or "alien sex toys" maximizer, is just one of many unfriendly maximizers, then maximizing "human values" is just one of many unlikely outcomes. In other words, you can't just say that unfriendly AIs are more likely than friendly AIs when it comes to cooperation. Since the opposition between a paperclip maximizer and an "alien sex toy" maximizer is the same as the opposition between the former and an alien or human friendly AI. Since all of them want to maximize their opposing values. And even if there turns out to be a subset of values shared by some AIs, other groups could cooperate to outweigh their leverage.