The way O'Brien writes doesn't really allow the reader to gauge any real truth about what happened. Or perhaps "truth" isn't the right word: it doesn't provide his readers with many facts. The better way to describe what we get from the author and The Things They Carried is to say that we get an impression, which is and always remains highly subjective and distorted.
That's the case with these two short stories as well, especially "Notes." Do not be fooled by the fact that the author seems to be explaining the history of "Speaking of Courage." Considering how much O'Brien talks about false memory, the impossibility of finding the truth, and war stories in general, it would be a mistake to take his word for it even in parts where he uses the first person perspective to speak to the readers. Nothing about "Notes" really explains "Speaking of Courage"—it just muddles the real truth further.
That being said, we also can't be sure that "Notes" isn't factual. Perhaps that is what truly transpired. In that case, "Speaking of Courage" didn't happen at all. Not the way it's written and presented to us, at least. Norman Bowker might have driven around a lake again and again, but the only real "truth" of the story would be the fact that Kiowa died a horrible death and that Bowker couldn't tell the story himself, despite needing to let it out somehow.
The only real parts are the guilt and the distance Bowker feels when he sees people living their normal lives. O'Brien says in "Notes" that he made up everything else: he condensed Bowker's experiences into a single afternoon, he invented and probably exaggerated the small town to be the sharp contrast to the war, and he put Bowker's guilt about Kiowa's death front and center. "Notes" ends with O'Brien telling us that Bowker actually wasn't responsible for Kiowa's death at all and that he didn't freeze and didn't lose the Silver Star. So that part is made up as well, according to the author.
The point of these two stories is that we aren't really any closer to the truth about what happened that night. I would say the only facts of the matter are that the soldiers did get ambushed on that terrible field and that Kiowa died. We can be sure that Bowker felt guilty about that, but the rest is a mystery. O'Brien is a self-professed unreliable storyteller. Where Bowker was that night, whether he had anything to do with Kiowa's death, whether he could have actually saved his friend or just felt like he should have done more, remain unknown.