I wouldn't be. They started working on this game a long time ago, i don't think a windows 10 map editor was necessarily in the discussion back then.
Maybe it was... Maybe it wasn't... There's really no way too know... I'm guessing it was OR they will be able to impliment it sometime shortly after release...
Either way, the idea porting the whatever Forge component they had in development to Windows doesn't take a stretch of the imagination. Studios port things all the time...