To be exact, what you're editing is an array of ten 20-byte Pascal strings (so length-prefixed) which contain the world's flags. A ZZT Object can set or clear flags which are carried across boards; this mechanism was repurposed to add the "SECRET" flag, which blocks editing with the built-in ZZT Editor.

Unless... you use the "?" key to open the cheat console, and type in "+DEBUG" as a command (in ZZT 3.1+, IIRC) - then it will let you open the worlds and edit them as-is.