Those outside the AOM will face the combined power of AOM if they attack. In other cases it depends on who defends the codec.

H.265 is an anti-example, because there patents were pooled for offensive purposes to begin with, while AOM pooled resources for defense. I.e. something like MPEG-LA exists to extort money, not to defend anyone. That's also why you got two separate pools there. Patent aggressors don't want to share, each of them wants to get all the loot for themselves.