How To Restrict Access To Workflow Models For A User Or Group
In AEM 6.4, When a user is on a page, they can start a Workflow by going to Page Properties and selecting Start Workflow. Next a pop up is displayed which list the available workflows the user can start as shown below.

I would like to hide certain workflows in this list from certain users/groups. For instance, I want to show only the Download Asset and Project Approval Workflow for a group. Everything else should be hidden.
In the following article, How to hide workflow models in the start workflow list?
It states that I should be able to add the workflow:system tag to a workflow model to hide it. However, adding that tag did not work.
What is the recommended way of restricting the list of workflow models that appear when a user selects start workflow?