Pretty much. I have only converted one iBook to ePub. It was the Swift Programming Language manual, and I suspect that it was an abnormality (e.g. no DRM, which is atypical for ebooks).

For what it's worth, the process looked like this:

1. Find where the books are stored. (They're stored in ~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books.)
2. Figure out which cryptic file name corresponded to the book. (Easy in my case, since I only had one book.)
3. Compress the contents of the directory using zip, ensuring the extension was epub.
4. Import the book into Calibre.

I wouldn't count on this working for any old ebook, particularly purchased ebooks, since this manual was borderline promotional material for a new technology that Apple was introducing. Yet it does demonstrate how complex the process is for the simplest case.