Yes, i figured it had to mean both since it was plural. But since both objects are created in the constructor explicitly - the assigned() checks will always return true unless the component is being destroyed (hence i figured checking for csDestroying would be better). But both horizontal and vertical scrollbars may not be visible, either one can be visible or invisible depending on the content.
In delphi this one is used to determine if there are any scrollbars visible during loading, assignment etc. I seem to remember that it's particularly called when cleaning up a view (f.ex in an imageviewer, this would be called to check if the scrollbars needs to be reseted prior to loading etc.)