The orthogonality thesis, in the form "you can't get an ought from an is", is widely accepted or at least widely considered a popular position in public discourse.
It is true that slow superintelligence is less risky, but that argument isn't explicitly made in this letter.
An article by AAAI president Tom Dietterich and Director of Microsoft Research Eric Horvitz has recently got some media attention (BBC, etc) downplaying AI existential risks. You can go read it yourself, but the key paragraph is this: