Perhaps you misread the ending bit (or else I did)... to me, it was part of the multi-faceted way that storytelling affects how we understand events and yet renders facts elusive.
Here's what I got from that reveal in the end credits. It was not to suggest once and for all that that dude WAS the real father, or even possibly the real father. Nor was it to suggest that Diane Polley was some sort of whore, as many here seemed to think.
My take was simply this: when they were kids, and teased about the possibility that this guy was probably Sarah's real father, that guy became the odds on favorite and even looked like her and it really seemed like it might be true. Yet, when she confronted him about this part of his past & how he felt knowing he was considered as a possibility, he downplayed it and said Diane and him had just been friends and there was no romantic attachment between them at all.... yet, he said it in a way that seemed to betray that there was more to his story than he cared to reveal. True enough, when pressed by Sarah for a straight answer (as well as removing doubt by admitting a DNA test proved another man was in fact the father), he finally came clean and admitted he DID sleep with her.
To me, it felt a little more incidental. Yes, it's a bit of a twist to have him say, "but wait, I DID sleep with her after all! Maybe I AM the father!" but if that interpretation exists, it's only as a small joke, as if alluding to yet another layer to this story. But the fact that the DNA test is 99.997% makes the question irrelevant at this point, right?
Point being, in keeping with the themes of telling the stories of our lives, we choose to withhold information and reveal information in such a way that the "truth" can be elusive. The way this guy behaved during this portion of the interview helped underline the theme of secrets and the narratives we tell to obscure those secrets from others.
On the other hand, this is just how I interpreted that ending. He was unwilling to present the truth to her and originally lied about his relationship, then later revealed a different version by admitting to having sex with her. Maybe he was lying then? Who knows. As you suggested, maybe nobody knows anybody all that well.
It was not to suggest once and for all that that dude WAS the real father, or even possibly the real father. Nor was it to suggest that Diane Polley was some sort of whore, as many here seemed to think.
That's a total WTF to me that anyone would interpret it in either of those ways. People are so weird, and gross.
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I haven't read the rest of the replies yet...but you completely nailed it, I think. Particularly the part about this moment SO illustrating exactly what the movie is about. Or at least one aspect...how the telling of our stories is so...conditional.
Anyway...it's late and you already said it so well, so I'll just say, great perspective. Nailed it.
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