Further details:
If I hard code some Author names to populate the drop down box (i.e. just type them in the settings dialogue) it works fine. But that is a nasty hack that is not really scalable as we would keep on having to edit the page definition every time we got a new author, rather than just upload the data. Things like this should be data driven, not coded.
I also tried to define a boring old text box for the user to type the name in. Wix refuses to filter on such a widget, the dialogue insisting that the thing to be filtered on must be in a dropdown. Who designs this stuff? Having the user type in what is to be used in the filter is a perfectly reasonable requirement. Hey, if I were designing this, I would allow you to define an entirely free format filter on the user side, so the user could select the field to filter on, and the comparison (equals, greater than, contains, beytween x and y, etc., etc).