I don't know the precise answer to the original question. But with regards to AppCode it's pretty much a standalone product that has features IntelliJ doesn't.
IntelliJ incorporates everything found in all the other IDEs like RubyMine and PyCharm but doesn't include the additional stuff found in AppCode.
I've heard that the reason for this is the XCode project format. The purpose of AppCode is to be a better IDE for iOS/OSX development. But if the future version targets cross platform development, not meaning cross platform on the host side, but cross platform on the compiler target side; it would probably mean that the project format support should support Makefiles and other build systems; and hopefully that would be integrated in IntelliJ.