Following the main help doc wasn’t working at all for me and based on the small details mentioned in the other link I suspect it wouldn’t work for others either.
Hopefully, some else finds this information and it helps them out.
In my experience, importing sites in bulk does work, i did alot of sites imported, with CSV file using tab divider : site name + site site url + username + groupname. Just make sure MainWP child plugin already activated on child site and make sure the site url is the correct url (http:// or https:// with www or non www).