Ang said that Lureen was lying. He says it's better to let the audience work for it.
When asked, Annie would say she doesn't know which way it was. She says we have to finish the story ourselves.
How are we mere mortals to know?
So, I think it's unknowable. And we'll just have to stand it.
I don't think we're actually supposed to know, because of what both Ang and Annie said.
The ambiguity is deliberate, in both story and film.
And, I think Ennis too would have to stand it; to live the rest of his life without knowing.
When he first hears about the accident, he immediately thinks of the tire iron, for sure.
That will probably always be his first reaction to any gay man's death.
But later, when the shock wears off, he may start to wonder. Why would Lureen lie to him?
I think there's a chance he will reevaluate his first assumption, at least later in life. And come to the conclusion
that he will never know for sure.