Welcome to the community forum. As @steevej notes, the Atlas restriction on Database Users is for accounts that log directly into your MongoDB deployment. Atlas Users are able to monitor and manage your Atlas clusters.
Using your original terminology, those are limits on admins rather than a limit on customers.
Direct access to your deployment should be limited to trusted database users connecting from a limited range of whitelisted IP addresses.
End users of your application (who are logging in and ordering items) should have accounts and permissions managed by your application or API.
The same user model should be followed for any database deployments. An application can scale to millions of users, but those are application users rather than database users.