If you are running Tableau Server 9.2, you are already running parallel queries. The linked knowledge base articles are how to configure the parallel queries, not enable them. I agree that they are a tad confusing - I wish it were more clear.
To pull a quote from the KB:
This value overrides the default parallel query limit. For the vizqlserver process, the default limit is 16; for the dataserver process, the default limit is 1000000. However, for both proceses, the default is 2 for Amazon Redshift.
So yes, the feature is activated by default without doing anything with tabadmin.
Is the user who requested the change experiencing any issues?
Thanks Nathan - I found the tabadmin set command defaults in the documentation also http://onlinehelp.tableau.com/current/server/en-us/help.htm#tabadmin.htm
They were just trying to increase the performance of a dashboard and read their performance recording as though queries were being passed serially.
I just wanted to be sure I wasn't misunderstanding the defaults.
In my experience with the performance workbook, the only thing you can rely on is whether you see overlapping "Execute Queries." Sometimes the queries are so fast that you won't see any overlaps. In this case, the absence of overlaps does not necessarily mean, parallel queries did not occur. Hope this helps.