I've always read the line in the light that since Harriet is no longer pining for Mr. Knightley, there's nothing for Emma to be concealing. The problem has vanished. I never thought that maybe she intended to tell the secret. I don't see what would be the point to that. It would be embarrassing for everyone involved, I think. I suppose it could be taken either way, but my reading certainly makes me happier with Emma. :)