It is "optional" in the sense that it removes hassle from the user. This is what the original article and my comment are all about. And don't get me started on building anything remotely more complex stuff for Mac from Windows or Linux. With the other too it's trivial to build and ship across by comparison.
I wouldn't call WSL hassle free for the user. Quite the contrary. And compiling stuff on Windows is a nightmare compared to Linux and macOS. It's become marginally better since you don't have to install the entire Visual Studio monstrosity anymore just to get a compiler that plays nice with the system. But it wasn't always like that. On macOS you could always install the necessary parts from Xcode without App store and activate it via command line.
