Not entirely sure why it's sold for 20$ knowing that the source code is freely available on GitHub [0]. What prevents anyone from building the distro themselves?
Nothing, you pay for the easy updates and to support the development. Paint.net does it the same way and probably some others that do open source on Windows.
Personally speaking, $20 is a bargain compared to the cost in personal time and effort to build and install it from source. I imagine that there's a sufficiently large demographic with enough money that $20 is basically free but not enough money to have a personal assistant to set things up while they do something else.
Poking around it kind of looks like a Linux distro in the truest sense. It's more a collection of freely available packages than anything else.