I don't get the ending

Well the full story behind it was that Ford had talked with the race organizers when it was clear that Ford was going to win in a 1-2-3 finish. The organizers agreed to let Ford slow their cars to get the marketing photo that Ford wanted, however after it happened the organizers basically went back on their word and pulled out the provision of how to determine a winner if their was a dead heat finish. The Ken Miles GT40 was 20 yards in front of the McLaren driven GT40 so it would have had to have been at least 20 yards ahead of it. I can't recall where the other GT40 started but there would have been yards needed for it as well. The race organizers were the biggest turds in the whole thing, not Ford as they thought Miles would have gotten the victory and from a marketing standpoint having their car used by the first driver to get the triple crown in endurance racing would have been pretty meaningful to them. Blame the French.

Or if you want to dwell even deeper into it you could also blame the pit chief for the McLaren GT40 as they used the brake rotors that Ken Miles' team had beaded in for his car on the McLaren which forced Miles to have to pit for more brake work due to poorly performing non-beaded rotors on their last pit stop. Had that not happened, Ken Miles would have been at least 2 if not 3 laps ahead of the McLaren GT40 so that even if they crossed the finish in a dead heat Miles would have still won because he would have been a couple of laps ahead in distance. There are a lot of things you could point to as the cause of him losing... certainly more than the film bothers to bring up.