I did a bit of trial and error and can confirm superflat does actually work, but those documents are out of date with generation. Here is a link to a working dimension file I can confirm works by testing.
I know it's a bit over the top, but it should help with information regarding this type of worldgen. https://gist.github.com/drhax9908/8c9dbfe4a72cc08c1ff4dd0015b748b0
EDIT: I noticed I said something twice by mistake.
I have this issue as well, I had it equipped but changed to my other cape, and the founder's cape vanished from my capes section. Oddly enough, I was able to find the skins that the cape has with it via a google search for the store item, and was able to equip the cape via the skins section, but not the character creator