What sort of argument is that? If the Duet Display author wasn't able to find the GPL license for CocoaSplit, how could he assume that the code was available for any use? Furthermore, the ISC and BSD licenses of PeerTalk and GPUImage are rather permissive, yes, but they do require attribution that is not given by Duet Display.

The big deal here is that licenses matter, especially when you're profiting from others' work.