Oh, I see. Looks like EditorSettings can be changed only in a tool script, so it would only be useful for developing a plugin. I think you have to declare a variable to get/set the editor settings:
var settings = EditorSettings.new()
Then to get a setting:
settings.get_setting("setting")
And same to set a setting:
settings.set_setting("setting")
I haven't really used this, but I think that's how it works.