The license doesn't indicate that you can get out of the terms by developing your own map editor. A map editor would qualify as a game engine product because it can edit game content and their license specifically prohibits you from redistributing a game engine product that's built with Defold.

If the map editor is an extension (and they have lots of example extensions on github, all of which I've checked are under normal open source licenses) rather than a set of patches to the core code itself, it isn't subject to the Defold License in the first place.

(so if the extension API is missing something, contribute the feature(s) you need back to core, then you can write your extension free of issues, so far as I can tell)