Did you ever learn the answer to this? I'm trying to set this up right now.
I realized after doing the Steam tax interview that since I'm in the US, I'm not required to do backup withholding. I asked Itch.io to let me redo it and the second time I filled it out, I didn't run into the problem I described in this post. :/ Don't really get it, but everything's in order now. Good luck with yours!!