Not true, f.lux released a version of their app in the App Store. Apple pulled their app and so they added another method to sideload it https://justgetflux.com/sideload/ using xcode. Apple then revoked their development license which broke that method as well. It's always been available as a jailbreak tweak but it's also been available to the general, non-jailbroken public
I don't think you understand how Apple works. They love to promote proprietary software in their "walled garden".
Apple's concern was that f.lux changes the color of the screen outside the app, and that fact alone goes directly against Apple's philosophy of keeping apps in their own respective sandboxes. Only Apple is allowed to change UI outside of an app. Consistency at all costs.
I think you don't understand what the comment was implying. If it was open source anyone could build it using xcode and install it in their devices themselves.
