We document that it is necessary to embed Tableau dashboards in an accessible web page for them to be accessible. The whole point of doing that is so that you can view the dashboard without the surrounding "chrome" from the Tableau Server or Tableau Online UI, which is not yet accessible. Often this works great as our customers frequently embed Tableau dashboards anyway, but sometimes there isn't another web page in which to embed them.
Is there another way to get rid of the offending non-accessible UI? You bet there is!
If you append the URL parameter :embed=yes to the URL of your dashboard, Tableau will render the dashboard without any of the UI chrome that isn't (yet) accessible. Then you're good to go!