It may not work on past posts, of which this is one because it was posted before you changed the setting, but it should work on future ones. The past posts don't change settings when you change the default, but the future ones do. I don't know if there's a way to change the setting for past posts - I looked at it to see, and if you edit, the same options don't seem to come up. There's an allow comments button that can be used to shut them down altogether, but no reset for enable comments. Which seems weird to me - what's the difference between enabling and allowing?

Maybe we really should all collectively abandon LJ for Dreamwidth.