If Trump wasn't contemplating an attack, and Milley told the Chinese we weren't going to attack, what's the problem?
A lot has to do with 1) context then and 2) how it is characterized now.
As to # 1, context at the time, the Chinese were genuinely and gravely worried that Trump would in a fit of egomaniacal rage launch an attack on them. Given that Trump was desperate at the time to delegitimize the election and Biden, and given that he bought into the far right conspiracy theories from his fanbase on this, it is not that far fetched that the Chinese would be concerned that Trump would attack them and use that to try to bootstrap in an argument that the election result was indeed flawed.
Evidently Milley saw this concern as not unfounded and, witnessing Trump firsthand, thought he needed to both reassure the Chinese and make sure that the people in the process did not yield to pressure from Trump calling on them to do something stupid militarily, and outside normal process. Recall, Trump in his state of anxiousness was showing a lot of willingness at the time to go outside the process for a lot of things. Again, in context, this was reasonable.
As to # 2, how it is characterized now, Milley is now viewed by the Trump cult as the ultimate traitor. From inside the administration he has confirmed the worst suspicions and concerns about Trump, which is a reflection on the rest of the Trump cult and the GOP as they are now fully coopted by him. Milley has confirmed that Trump is not fit to be President. And so despite the gravity of the situation their response is, not surprisingly, to lash out at the Milley as having done something wrong by acknowleding now what really occurred then.
And so calling the Chinese and reassuring them that things were under control and the regular processes were in place has turned into characterizing it as Milley promised to give them advance warning of any attack. It is very hard to believe it was exactly as the Trump cult is now spinning it for their attack on Milley. So we really need to know exactly what he said to calm their fears and reduce tensions. For example did he merely say that the controls were in place? Did he say that if there was cause for concern he would convene the principles and try to address it? Did he say he wanted to keep open a channel for deescalation?
The bottom line is this. For the upteenth time a former Trump official or associate has told the truth. And for the upteenth time he has been attacked as deserving to go to jail for daring to tell the truth. Regardless of the specifics of each of these instances, there is a pattern here that ought to signal to the cult members that, hey, maybe there is something to this complaint that Trump has no business being anywhere near 1600 Pennsylvania Ave. and is better left to be the carnival barker he is and to just be used generate some funding for GOP causes.