In my neck of the woods (that means area where I live and has nothing to do with wooded areas), the phrase means the same thing and I interpreted it as such and that the asshole started to pop off on her (that means suddenly start talking angrily rather than whatever your mind may have conjured if you hadn't heard the phrase before).
It's very likely a wish fulfillment fantasy story, why would it have involved the asshole only walking towards her (which is possible to do with obvious intent, but still ... what would he be apologizing for later in the story?) when you can literally make up anything you want?
I think I interpreted your "stretch" comment appropriately ... If you meant that Catoblepas was literally becoming a Mr. Fantastic like superhero, then I have no idea how to respond to what your said and I apologize for the wall of text that makes no sense in context.