It means the other engines are less "open"?

Sort of a subtle dig here -- What I mean is game engines _of this type_ where they have a whole IDE and everything tend to be so insular that even if they are open source, it tends to be hard to use parts of it without buying into the whole ecosystem. Maybe this is the nature of the beast when it comes to large game engines, I've just always dreamed of something modular that lets me pick what I want to use, and godot wasn't it either

"Open-source" means nothing alone, what matters is the licence. For eg. Unreal Engine is "open-source" but under strict license. Defold engine is also "open-source" but has a Apache-modified custom licence preventing the commercialization of editor derived work (so no commercial custom editor), etc.

There are many degree of openness and Godot goes full MIT so Godot is truely yours (both the editor and the engine)