Except the president has extremely broad powers when it comes to handling classified information. No one actually thinks what he did is illegal, but just a further symptom of his incompetence. The harm it could do to our allies and our ability to share classified information is what is important about this.
What I found most funny is that McMaster didn't actually say it was untrue. He said the story as reported was untrue. If you listen to his statement it is a veritable dance on a tightrope to not confirm nor deny anything.