More than that, they were distributing an Xcode project that was little more than a binary, using the recent changes to developer code signing in Xcode to get that binary on to user devices. I can see how that alone would provoke a reaction from Apple, private APIs or not.

After all, there are plenty of open source projects on Github that merely use private APIs, the browser for tvOS[0] for example.

[0] https://github.com/steventroughtonsmith/tvOSBrowser