Just randomly ran across this which says he didn't sign the letter, even though Ilya Sutskever is on there, and remembered this thread:
https://www.youtube.com/watch?v=VihA_-8kBNg&t=17m35s
So I went and checked the letter. Here is the letter with signatories, I don't see his name in the 'J' section (alphabetized by first name):
I don't see a longer name for him that doesn't start with Jan anywhere else I could find. I think that tweet was the same day as the letter though, so maybe he's on there and I just didn't see it, he meant something more complicated and didnt agree with all of the letter, or he missed the deadline for signing the letter.
https://twitter.com/janleike/status/1726599944029753750