When you create a group of custom fields, they are displayed by default on the event info page.
What matters is the name you give when creating the group, once created, you can rename the group later (eg. to "internal info") and it will still be only visible from the back office.
All the other features work the same, these fields are still visible from the reports and other backoffice screens. The only thing the extension does it to not display the group on the info page.
existing groups
We haven't found the budget to improve the UI to change that (hint, hint), but as a workaround, you can either change it directly in the database or use the api explorer to alter the name (the later is safer).The entity is CustomGroup, change the name (only lowercase and underscore, it has to start with "private")