He lacks humility about minds that humans cannot comprehend
"That whereof we cannot speak, thereof we must remain silent" ~ Wittgenstein
I'm personally not terribly worried, as I think we have a ways to go before creating human-like general intelligence, let alone super intelligence. It is also not clear at all to me that an exponential increase in intelligence is likely or feasible after some threshold. Still, treading carefully seems prudent. There are some serious risks, and even if the likelihood appears low, there are a lot of unknowns.