Since I imagine there are many HN readers that might not get it: "The King in Yellow Borders" is a ZZT inside joke. When editing a ZZT world, new boards by default are created with a yellow border of what (if memory serves me) in Unicode would be U+2593 "dark shade" blocks. A game containing boards that maintain these borders in an unironic way would generally be perceived as... well, I'd say "amateurish" but ZZT creators were generally all amateurs, so let's go with "unrefined".

(It's also a pun on The King in Yellow but that reference is explained in the itch.io page.)