> I don't think Photoshop is a good counter-example since OSX has top-tier graphics applications AND runs on Linux.
No, OSX runs on the Darwin kernel, not Linux. For a number of reasons, you can't take an app designed for OSX and run it on a laptop using Ubuntu (or another Linux distro).
To be fair, it would be rather trivial to port the upper levels of OSX to any other Unix-like OS. Darwin exposes a very BSD-like interface to applications.
Apple considered, for some time, replacing classic MacOS with Linux (I have used Apple's MkLinux a lot).
No, OSX runs on the Darwin kernel, not Linux. For a number of reasons, you can't take an app designed for OSX and run it on a laptop using Ubuntu (or another Linux distro).