Yes, by default it will create extract for data source because whenever you develop any report using Tableau public and publish it onto the Tableau Public online, it wont connect to your local data source or RDBMS.
Because anyone can see the report that developed by anyone using Tableau Public.
For more information please look into the below link for FAQs:
Tableau does extracts data source every time you connect to the new data. There is no live connection option in the tableau public.
Thanks and Regards,