Well, that's the theory. It is true that they eat acros in the wild. Stomach content analysis on wild OL has found acro polyps in the gut. Is there something magical about coral flesh that means that the filefish can't survive without it? Or, is it just that they are such poor hunters that coral flesh is about all they can get? Mine haven't seen coral flesh in over eight months (and have been in my care since New Year's Eve), and I don't believe that coral flesh is anything special. It's just flesh. So, the key is to get them eating alternate foods. Once you manage that (and don't get me wrong, it was _very_ difficult with these fish), they are easy to keep. They spawn for me on a regular basis.
By some accounts, those plebius butterflies are also obligate corallivores. Same deal with them, since they also have no corals in their tank for over six months now. They grow really slowly, though, and were _tiny_ when I got them.