They didn't "theoretically" open source it - they
actually open sourced it.
I get why people used to shit on Microsoft, but Microsoft has demonstrated over a number of years that its changed under Satya.
> However the way that this always works is that there is a base that can be written in, but anything non-trivial will have pulled in something that, surprise surprise, is Windows only.
Outside of desktop GUIs, this is simply not true. I'm writing complex, cross-platform systems that work just fine on Windows and Linux (and would on MacOS if I chose to target it).
Hell, even a lot of the tooling is now cross-platform: Visual Studio Code, Azure Data Studio, even Visual Studio and Xamarin run on MacOS!