Following up, no, it’s not just the “Enable User Creation” setting. Changing that setting, creating a new team, and trying the import again produces the same result.
… and now I have two junk test teams. How do I delete them? I tried
./platform -permanent_delete_team -team_name="foobar2"
It asks if I backed up the database (I did), there’s a brief pause, and the team remains in the database. Just not my day I guess…