I've removed Rosetta 2 from my machine. I then downloaded the Apple Silicon Spine installer and can reproduce your issue. I see if we can fix it, but I wouldn't hold my breath. We have no control over the installer generator.
That said, uninstalling Rosetta 2 is pretty easy. I installed Rosetta, installed Spine via the Spine Installer, then removed Rosetta 2 again and ran the Spine Editor just fine.
edit: Looks like the generated Spine.pkg has a Distribution
file that is missing the <options hostArchitectures="arm64,x86_64" />
key. I'll see if I can make this work with our .pkg generator. Thanks for the heads-up.