In line with Matthew’s comment, think it’s true that several sources discuss the possibility of unknown risks or discuss the total risk level (which should presumably include unknown risks). But I’m also not aware of any sources apart from Ord and Pamlin & Armstrong which give quantitative estimates of unknown risk specifically. (If someone knows of any, please add them to my database!)
I’m also not actually sure if I know of any other sources which even provide relatively specific qualitative statements about the probability of unknown risks causing existential catastrophe. I wouldn’t count the statements from Leslie and Bostrom, for example.
So I think Michael’s claim is fair, at least if we interpret it as “no other sources appear to have clearly addressed the likelihood of unknown x-risks in particular, which implies that most others do not give unknown risks serious consideration.”