I disagree that there's no reason to believe it will ever exist. For one thing, many smart people are trying to build the technology right now and they believe it to be possible. I see no compelling case that the intelligence scale simply tops-out with humans; that a more intelligent system is ruled out by the laws of physics.

The topic here is human extinction caused by AI. I don't know of any serious argument for why a non-general intelligence (really a system less intelligent than a human) would pose an extinction risk to humanity.

Plus, my background understanding of the people who signed this is that they're worried about AGI, not present-day systems, but that's an inference.