Except that you can't build anything for OSX when you don't have a Mac. That's a totally artificial constraint. Not lock in, you say? Hell,_Microsoft_ is more open than this... the crowd favourite lock-in bogeyman no less...
Also, you certainly can develop for macOS from another platform, if you're using a development environment that supports this. There's a number of cross-platform environments that will work. The latest fad is Electron. You just can't build a native Cocoa app. That said, I'm unsure if any third party has a way to codesign your macOS app or if that's limited to Apple's developer tools.