Hi @rugged, yes, that is correct, by default the other fields will be ignored. The customization need to be made at Mailchimp side and additional fields added in Webflow. Webflow does not control the information saving flow at Mailchimp, we just pass the information from the fields given to the mailchimp script. 
Glad to hear you got it working Another way to do it is using our Zapier integration to Mailchimp: New Feature: Zapier integration for Form Submissions - Webflow announcements - Forum | Webflow
Cheers,
Dave