Large parts of the OS are open source though. Not just the Linux kernel and userland, but also some specific things:

Sailfish: https://github.com/sailfishos Android layer: https://github.com/libhybris/libhybris

Yes, it's how they make money as a company and pay their developers. Otherwise, we wouldn't have the choice of Sailfish OS instead of iOS or Android.

Proprietary is not necessarily bad.

... and yet it's a tonne more developer friendly than Android is becoming.