Good find!

It's not a doc bug, it's a product bug - a regression
introduced in Emacs 29.  In prior versions it works
as the doc says: (dired-get-marked-files nil 'marked)
returns the file name on the current line.

Wait, so dired-get-marked-files isn't supposed to be able to distinguish between

a single explicitly marked file and no files being marked?  But that's information

that I need.  How do I get it, if not with dired-get-marked-files?