There's nothing inherently wrong with the CJCS calling a foreign counterpart and telling him, "Relax, dude, no one's going to nuke you." That's not treason, it's diplomacy. If we do plan to nuke China, it's strategic deception, but diplomacy is more likely.
What makes it problematic is the context and what that tells us about Milley's mind. Between his childlike acceptance of anti-Trump conspiracy theories and his focus on "white rage" while the security situation in Afghanistan deteriorated, he's clearly not focused on his job and probably subject to manipulation. This doesn't mean a foreign actor is manipulating him, though it's possible. It could be a domestic political or corporate actor, or just the miasma of bad ideas that float around Washington and the executive class generally.
All we can say or sure is that if a Trump loyalist did this in 2016, he and his family would have faced prison time.