But he was the Commander of US Marines in WWI. One of the few times that has occurred.
Marines were 'under' MacArthur in WW2 and Korea. Marines were under Schwarzkopf in the Gulf War. What is rarer (?) is when the situation gets reversed -- when a Marine officer has army units serving under him. But now that Marine generals are getting into the rotation as joint commanders it will no longer be as novel as it once was.