Thanks for the debug man, hopefully that'll help someone in the future.
The only thing I can think of without looking into the loader class is this:
1. The config is autoloaded into the config array
2. Ion Auth tries to load the config file into the config array as config['ion_auth'] but the loader doesn't load it since it is already loaded.
3. The identity column is being referenced as config['ion_auth'] because of the second parameter but it can't find it.
That's my best guess without checking into it more...