Sailfish is not proprietary.
Lipstick (the UI) is proprietary. The OS itself is not.
That said, as someone who uses a SailfishOS phone everyday, I both wish Lipstick was OSS and wish this Commodore thing didn't exist. It's only going to bring poor publicity to Sailfish due to its stupid pricetag and featureset.
I don't have admin rights at work so can't install WSL.
But for whatever reason, Virtualbox is something I'm allowed to install. So I did. Now I just use Debian for everything via Virtualbox, IT dept be damned