looks like you tried to import a MBOX file type 9 years ago.

import mailbox
mailbox.mbox('path/to/archive')

This type of file is not supported, according to this migrating instructions, or perhaps it changed since your trial. To import a MBOX file of course would be much easier than to import several thousands of EML, one after another.

The API instructions are not helpful.

Perhaps, if somebody reads this now, it would be very cool to find out: does the 'Groups Migration API v1' support MBOX as a import file?

I had no success.