Allow one user access to hikashop module

The demo user on our demo website is configured to have access to everything on the backend of the website.
That allows you to access the HikaShop configuration and set the access levels of your users groups via the 'access level' tab.

That's what the post of that thread was saying, and not that the demo user itself is restricted to access only some part of your website backend.

If you want to restrict access to some part of your backend to some user, you can assign that user to a specific user group in joomla and then configure the access levels of that user group in the joomla configuration.