From there, further export formats would be available. You could probably (manually) ‘select all’ inside of each note and then copy/paste into a MS Word document (which is now an open standard).
Your best bet is probably either some sort of scripting to extract the content while OneNote is running (I’m not sure if the Mac version has AppleScript support), or processing the raw file itself.
The lack of export is a problem it unlikely to be fixed for technical reasons.
