Your understanding is correct. Tableau Online cannot connect to a BW data source natively. You'll have to put the data into some kind of intermediate data store for Tableau Bridge to use.
Alternatively, you could use Tableau Server instead. But that is usually a higher cost option (both up front and from a long-term maintenance perspective).
Alteryx can connect to BW and output a TDE or Hyper file for Tableau to use, or populate a relational database with the extracted data.