I read Luke as making three claims there, two explicit and one implicit:

1. If science continues recursively self-improving AI is inevitable.
2. recursively self-improving AI will eventually outstrip human intelligence.
3. This will happen relatively soon after the AI starts recursively self-improving.

1) Is true as long as long as there is no infallible outside intervention and recursively self-improving AI is possible in principle, and unless we are talking about things like "there's no such thing as intelligence" or "intelligence is boolean" I don't sufficiently understand what it would even mean for that to be impossible in principle to assign probability mass to worlds like that.
The two other claims make sense to assign lower probability to, but the inevitable part referred to the first claim (which also was the one you quoted when you asked) and I answered for that. Even if I disagreed on it being inevitable, that seems to be what Luke meant.