Hi @mclement,
Welcome to the forum!
I just updated the forum thread you shared here: NO BUG: Limited Editor can't add new accordion items - #4 by charaf.
As I mentioned there, “Edit content” is for editing content not adding new content which is why you can’t add new controls. I do however see how having control over whether or not this is possible can be useful when you’re handing this off to a client so I’ve categorized this as a “Feature request” and also mentioned it to the team.