on your outlook. If you tend to be a realist, you will go with the notion that he is not Sean and just read the letters and/or did some research to pull off the charade. If you are of a more mystical outlook, you will tend to believe that he is the reincarnated soul of Sean.
The movie, as so many do these days, leaves the interpretation up to you. Having just seen the movie I am currently at 50/50.
Schrodinger's cat walks into a bar, or doesn't.
You probably believe in Santa Clause, the Tooth Fairy and God too.
shareThis just isn't true. Contrast this with a good movie, like Elle, where we really are left with ambiguity: what really happened, what was Michele's motivation, etc.... the more we think about that movie, the better it seems.
But in Birth, the opposite happens: the more we think about the movie, the worse it seems. Glazer does give us the answer, but it just isn't believable (see the many threads on it here). There's a superficial air of mystery that rouses our interest, but the "ending" is just a cheat. The same thing is true of his later effort (Under the Skin). A resolution that just isn't believable. Maybe this is because Glazer is a music-video director. The plot doesn't matter, just the atmosphere. Bah!