The narrator of The Sympathizer is a double agent. Though working as an aide-de-camp to a South Vietnamese general, he's also at the same time relaying intelligence to a close childhood friend who's an operative with the Communist North Vietnamese forces.
Among other things, this means that we need to approach the confession the narrator subsequently writes for the North Vietnamese with considerable caution. After all, this is a man whose trade is treachery, a man for whom lying and duplicity are almost second nature. This doesn't mean that the narrator's confession is a total fabrication from start to finish, but it does mean that we cannot regard him as completely reliable.
Right from the beginning, we're put on our guard concerning the narrator's reliability. He tells is that he is "a spy, a sleeper, a spook, a man of two faces." Not only that, but he's a man of two minds, able to see the point of view of both sides in the Vietnam War.
On the one hand, this ability to see both sides of the conflict lends a certain objectivity to the narrator's account. As he has understood and, to an extent, internalized the rival positions of the North and South Vietnamese, he can give a fuller account of the conflict, an account that transcends mere propaganda.
On the other hand, however, we must recognize that the narrator is giving his account by way of a confession extracted from him under duress. It is invariably the case that such confessions contain what the relevant authorities want them to contain, and so we must exercise extreme caution in assessing their truth value.