"I don't understand. If I write a hello world arc program, are you saying that's part of arc?"
For the sake of compatibility with Arc, yes. An implementation claiming compatibility with Arc should be capable of running programs designed for Arc 3.1.
"what do you consider to be the core of arc?"
We were discussing what constitutes "part of Arc" for compatibility purposes. "srv.arc" is not a part of the core of Arc, nor are any libraries written in Arc. But it is still "part of Arc" for the sake of compatibility, in the sense that a compatible implementation of Arc must be capable of running them, warts and all.
Unfortunately, that means that certain programs may end up relying on things inherited from Racket, such as the |...| syntax. I think it must be decided on a case-by-case basis whether compatibility with Arc 3.1 is worth it or not. In the particular case of |...| I don't think it's worth it. But I'm not the one implementing Arcueid: that's up to dido to decide.