Unity can export Xcode project with IL2CPP for iOS from any editor (windows, mac, linux).
So it can generate IL2CPP for iOS on any platform.
Why it can’t do so for Mac on Windows?
Compiling IL2CPP is a known problem and require Xcode.
Does generating IL2CPP also require Xcode or other Mac-only SDK?
Same question applies to generating Windows IL2CPP on Mac.
Trying to answer my qustion I come to this conclusion:
Build pipeline for standalone platfoms initialy designed to produce complete (runnable) builds.
Export capabilities were added later. So no one probably consider the need to split generation and compilation steps. iOS on the other hand initialy require two separate steps and support for that is backed into pipeline.
Being able to export project from Unity from any platfrom and then compile it on different machine using native tools (without Unity) would improve flexibility and ease some CI/CD tasks.