That's a little bit inaccurate: he believes that it is humanly possible to acquire a body of knowledge sufficient to align an AI (i.e., to aim it at basically any goal the creators decide to aim it at), but that it is extremely unlikely that any group of humans will manage to do before unaligned AI kills us all. There is simply not enough time because (starting from the state of human knowledge we have now) it is much easier to create an unaligned AI capable enough that we would be defenseless against it than it is to create an aligned AI capable enough to prevent the creation of the former.

Yudkowsky and his team have been working the alignment problem for 20 years (though 20 years ago they were calling it Friendliness, not alignment). Starting around 2003, his team's plan was to create an aligned AI to prevent the creation of dangerously-capable unaligned AIs. He is so pessimistic and so unimpressed with his team's current plan (ie., to lobby for governments to stop or at least slow down frontier AI research to give humanity more time for some unforeseen miracle to rescue us) that he only started executing on it about 2 years ago even though he had mostly given up on his previous plan by 2015.