Sure, you can also have iOS binaries forward relevant iOS SDK symbols to macOS. That doesn’t mean they’re not different SDKs with different limitations, and not everything that is on the store can be patched to run in the simulator because there are legitimately symbols missing or with stubbed functionality.

It’s literally a different toolchain and SDK.

You're not forwarding to a macOS SDK. It's the iOS SDK. Enough works for most apps to run almost perfectly.