The question and answer you reference is from 2015...a lot has changed since then!
You have workbooks in Project 1 and you are going to move them to Project 2. You want the permissions of Project 2 applied to the newly moved workbooks. To do so, simply make sure that you have the Project 2 content permissions set to "Locked to the project" (image from Tableau documentation):
If Project 2 is already set to "Locked to the project" then there's nothing you need to do after moving the workbooks since they will automatically assume the Project permissions you have set when they are moved.
If the Project 2 content permission is set to "Managed by the owner" but you still need the moved workbooks to assume the Project 2 permissions then simply set the content permissions for Project 2 to "Locked to the project", Save, then set the content permissions back to "Managed by the owner", Save. Note that when doing this, any custom permissions for workbooks already present will be lost as the workbooks will assume the Project 2 permissions. Unfortunately you cannot pick and choose which workbooks should be locked and which should be managed.
That worked, thanks
Glad to help