I've read Learned Optimism, Seligman at no point makes the recommendation that overly optimistic people should be less optimistic. He doesn't seem too concerned with accuracy, he just says that optimism will make you happier and more productive, so you should pursue it.

He comes very close to explicitly stating that mild optimistic self-delusion is good, because it will make you happier.

He acknowledges depressive realism, that is, the finding that depressed people make more accurate judgments in certain situations, but doesn't really address it.