It the source is out they can't hardly enforce anything or much. People will just go to github and compile it for free.
I think open-source-ux was talking about software where the customers would be companies, where there is comparatively less of an appetite for using pirated or unlicensed software.
Sure, there may be some individuals who will refuse to pay, but companies will not risk legal violations by not adhering to the licence conditions for the app.
In fact, the 'source-only' model is nothing new. Back in the late 1990s, developers used to sell UI and data components for Delphi with full source code. Even though code wasn't published to public code repositorities, there was nothing to stop individuals posting those components to the internet for others to grab without purchasing. Even so, developers were able to successfully sell their 'source-only' components to other developers and companies.