That is true. I had never come across this before, but you are correct. I suspect it was left out because there have been (a long time ago) disputes over whether source references are relevant on media objects. Some people (not me) think the media object is itself a form of “source”.
Currently that autocomplete field for citations only references matches in INDI and FAM records. I will investigate adding OBJE to this.
This is now fixed. You can download the zip folder attached to this post, extract the file ‘autocomplete.php’ and upload it your your web server, replacing the file of the same name in the root kiwitrees folder.