As a follow-on from my previous post, there is either a failing here on the permission setup or a failing on the user group promotion working as expected. So, let's break it down into two bits to find out which. I suggest we start with where I believe the issue is, and that's with permissions (which therefore isn't anything to do with this add-on).
Permission issue?
Your user should have the Registered group as their primary group. Any other user groups should be set as secondary. This is the correct way of doing things in XenForo and follows the principles set in this guide:
https://xenforo.com/community/resources/implementing-permissions-across-multiple-user-groups.358/
If you don't have it set like this, then it's likely a permission issue and I suggest that you re-do your permissions to work like this.
Assuming you do have it like this, then for the node you want to limit, you should have the various posting and editing permissions set to Revoke in the node for the Registered group. Then for the "special" group you are going to use for promotions, you should have these various permissions set to Allow.
So, assuming you now have everything set like this:
Can a user who is not in the "special" group post in that node?
If it's yes, then there's still a permission issue (which isn't related to this add-on) that needs sorting. You'll need to use Analyze Permissions in the ACP to determine where the issue lies.
If the answer is no, then all is well and we'll then look at the user group promotion aspect of this problem.