Indeed, they made Level Editor available. In fact, there was even a friggin' book on level design: https://www.amazon.com/dp/0782118690

Blast from the past indeed. Many many moons ago, I came up with a "bright" idea of designing a mod for Duke3D that modeled the office of my then-employer (large fintech company). At least I had enough sense not to boast about it - several months later a crazy guy brought a shotgun to HIS former employer and made national news, I don't think people would have taken my creation as lightly as I did back then. Still, working on it was fun.