You’ll have to forgive me, I’m not an attorney but I’ll give a spitball answer. My understanding is that under the legal theory of “merger doctrine” the original charging document (as in this case) alleged Murder in 2nd degree. As a result all lesser offenses (3rd degree murder, manslaughter) are automatically indcluded even if they are not specified in the original indictment.
Under this doctrine, then all lesser offenses can be included in jury instructions as to finding of guilt on the major charge or a lesser charge. And it becomes the responsiblity of the defense to argue (theoretically at least—or be prepared) all of the lesser charges. Even if they don’t come up in court.
Anybody feel free to correct me.