Maybe you should read your own link. Calley's orders were clearly illegal, 100% illegal. His men should have refused those orders. If any of them were unsure, they should have made a note of that, told other superiors in their chain of command like NCO's and such, etc., of their reservations. Calley knew the order he was given was wrong and he could have ignored it but he didn't.
That isn't the case here with Milley. Milley was an advisor to Trump, nothing more. He had no command authority, and his personal opinion of Trump is irrelevant. His personal judgement of Trump doesn't give him the authority to take command and withhold information from him. Doing so on his own volition is a violation of the UCMJ.
There was no case of Trump saying something like "Give me the nuke codes so I can start a war..." There was nothing like that. Milley just didn't like Trump's style and that's no reason to not do his job professionally. Milley was 100% wrong here and should be fired for it.