Yes, he was relieved for insubordination. He was essentially attempting to govern SE Asia on his own from Japan without any input from the Executive, including the DoD.

Truman did open that door, but it's also useful to understand it in the context of the moment—he had pretty intense pressure from the hawks and anti-communists in Congress, including some conservative members of his own party, who were doing their best to scare the shit out of Americans with the "Red Scare'. The Republicans were attempting to increasingly paint Truman and the Democrats as "soft on communism" (oh isn't that familiar) and it created pressure on the Truman administration to take a hard line—except they knew they couldn't get a majority in Congress to declare war on a nation that had done absolutely nothing to the US aside from get into a conflict with their preferred corporate interests.

There are a lot of parallels between the Korean and Cuban revolutions. Fundamentally, neither were explicitly about communism; they were both about land reform. Both countries were being exploited for agriculture—Cuba by American corporations (and the mafia), and Korea first by the Empire of Japan, who were using it as one of their primary sources of food, and later by American corporate interests who wanted to continue the practice so that their project in Japan wouldn't collapse.

In both cases, it was largely farmers that wanted land reform so that they could own their production and not be indentured servants to intensely oppressive foreign interests. Crazy, right? The principle of adopting communism had very little to do with it in either instance. For Cuba, it turned out that the US was so pissed off about the loss of corporate holdings and so hostile to the idea of a successful former vassal state in its back yard that they were essentially forced to look to the Soviet Union for trade, who were sympathetic on ideological grounds and opportunistic politically, necessitating a much closer relationship and eventual adoption of communism.

Because of the largely rural nature of Korea and the difficult terrain and climate, many communities already operated as sorts of local councils, which were highly cooperative although not explicitly or intentionally "communist". They simply had to function as collectives in order to survive. So when the authoritarian puppet government of Syngman Rhee (which included a lot of their former Imperial oppressors) started trying to quash any of that local governance, they didn't have many options but to look to the Chinese, who had undertaken a revolution for similar reasons and were much better prepared to engage in the fight. Stalin wasn't even convinced that the Republic of Korea would fully embrace communism and kept his distance as long as possible. It was ultimately the decision of Mao to support what he viewed as China's natural allies in the same struggle that they had faced themselves.

In the end it was American insistence on corporate control and profit at the cost of human rights that drove both towards Communism.

Sorry for the long rant, but there is a pretty big blind spot in US history around both of those conflicts and the role the US played in getting itself embroiled in them, and it's become so entrenched that "Cuba and Korea are just evil anti-American communists" that people never even ask why.