> Attention! Installing cmus-osx will prevent you from opening iTunes until it is uninstalled!
Since macOS 10.12 launchctl can not manipulate system services while System Integrity Protection is engaged.
Long story short: iTunes will always be launched when a media key is pressed unless it is explicitly disabled.
I take it that they're not using MediaPlayer.framework (https://developer.apple.com/documentation/mediaplayer), which was introduced in 10.12.1, that solves this exact problem? I would suggest against installing this until this is implemented.