There is no way to turn off the default recursive-ness of adding a folder via AppleScript (I’ll consider that a feature request though).
However, you can add individual files to the AppleScript command, and not just folders. So if you can make a list of the top level items in your folder, and send those along instead of the folder, then Retrobatch will do what you’re after.
Here’s a little technical insight into what’s going on inside RB if you or anyone else is curious:
When the AppleScript command is sent, RB gathers up all the items set to it and loops through each one to see if it’s an image file or a folder.
If it’s an image, then it’s added to a special list that is then turned into a virtual “Read Individual Files” node which is prepended to the workflow graph.
If it’s a folder, then for each folder RB encounters a virtual “Read Folder” node is created and prepended to the graph.
Then it all executes and the original workflow stays as it is.