They meant that they wanted to do a test to see if they would get any gpg-encrypted emails from people who saw the hat in real life; the “experiment” doesn’t work if you allow internet strangers to email you too, as then you don’t know where a person may have gotten the email address/key from
I’m not sure about markdown but plenty of IDEs can fold eg a method or a class. Off the top of my head I know Geany, Jetbrains, probably most gui text editors/IDEs do. Not sure about markdown specifically, I only use vim for markdown which I know can fold.