Yes, the username is the same on both computers. However, the path is different on Linux:
/home/gadgetboy/Documents/Publii/sites
vs Mac OS:
/Users/gadgetboy/Documents/Publii/sites
It looks like I can edit that in:
/config/app-config.json
…and that worked.
Thanks for pointing that out. I expected the path to be relative to the user directory.