I can confirm currently that keyword would print a special link that would lead user to a "event edit" view of the calendar page, which basically just prints out the original details screen of the event asset in the calendar page. You can not customise the editing interface, it's just the same editing interface in backend.
I think it's a legacy feature and it isn't ideal anymore. we have Edit+ and customisable simple edit interface now, it's much more flexible to use /_edit link of the asset instead, so it should just pop up a new window with the /_edit url of the event asset.
We can either introduce a new keyword which prints a /_edit link of the event (which is safer for backwards compatibility), or we can just change the code for that same keyword to print new link.
I'm leaning towards latter, i think it should be fine we just make that change for Matrix 5.1.