EditorLevelLibrary is under the title “Additional Scripting Utilities for Unreal Studio Users”. So if you’re using Unreal Engine without Unreal Studio you won’t have access to those methods (from what I could find). I barely know anything about Unreal Studio so I can’t tell you what it is, I also have no idea why Unreal Studio has better support for Python scripting than what is available in Unreal Engine.
So if I’m not wrong then the answer to your question would be that EditorLevelLibrary does not exist under Unreal Engine but exists under Unreal Studios.