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And if you're doing ruby or nodejs or a handful of other languages, you can use the XCode CLI and most of your code (possibly all) will never see the App Store submission queue.

Hence commercial.

It makes sense for Apple though, because what the devs are really paying for is access to Apple's closed-source ecosystem, not the tooling itself. This company has no such market power; why would anyone want to pay for Zen, when it has no established ecosystem or audience?

Signing binaries and distributing apps on the MacOS and iOS app store is commercial.

The compiler isn't commercial, unless you are saying that anything Apple makes is "commercial" because they are a for-profit business.

I'm writing a Swift project right now on my Windows computer using WSL, not paying anything to Apple...

Xcode dev here, I have a certificate with a free account.

You only need to pay for it if you distribute your app on the app store.