I don't see that as a lapse in logic, just a character not stating the situation as clearly as he ought.  Obviously, if the guy simply relived the other fella's last 8 minutes as they occurred, there'd be little reason for sending him back, and no reason for a movie.  So it doesn't seem like a lapse in logic to me.  I thought, when I saw it, that they shortcut the "scientific" explanations simply because too much detail would lose the audience, and too much detail would get in the way of what Gyllenhaal could or couldn't do.  As it was, they simply shortchanged the explanation and let us -- and the character -- figure it out as we went along.