What Milley did was 100% treason. There is a reason the president is trusted with such a significant chunk of the authorization needed to launch a nuclear strike - because in that event, you really don't want to have a chain of command scenario where you're calling back and forth amongst multiple people to figure out who is actually in charge. So for that reason authorization is traditionally kept compartmentalized but in a very limited way, the president and one other official are what is needed at core.
What Milley did was essentially assure a hostile foreign power that he would be willing to disarm America's highest levels of defense in the name of politics, and he warned Chinese military command of this. This is absolutely treasonous and in any functioning society he would be summarily executed following a hasty court martial seeking to avoid further embarrassment or risk.
The depressing thing is, he won't get punished for it. If anything he'll be rewarded. Military justice does work independent of the doj but the high command is all staffed with party loyalists who would personally piss on the Constitution if they received enough yuan to do it.