I’m not sure about what you’re seeing or that other post, but I have a repeater I just tested with and with or without, it also depends on the layout option set for the repeater. Displaying as a row, you can collapse the repeater even without making a selection. It is when you choose the table or block layout that you must select a field to show when the repeater is collapsed. It could be a bug, I don’t know. If you want to report this as a bug you should submit a new support ticket https://support.advancedcustomfields.com/new-ticket/
Actually, you are 100% correct. I just did some testing. It appears that there is a bug in the system that let me do this, and it is a bug that works for all layout types for repeaters.
Steps to recreate
1) Create a text field in the repeater, doesn’t matter what you call it
2) Set this field as the field to show
3) delete the field you just created and selected
4) save the field group
This needs to be repeated whenever you modify the field group, and it’s probably not a good idea to depend on bugs, but I’m guessing that this one isn’t going to be corrected any time soon, especially if no one reports it since it not breaking anything.
I was curious, so I did some testing. Hack #2, add a load field filter to any repeater field.