The press release tells you exactly how this works—the film 'probably wouldn't have happened' without Helena Hufnagel's grandmother, not without Amir, not without the grandmother's life and her moral awakening.
Amir is the real event, but his grandmother is the inspiration. This is the grammar of white saviorism—it works so smoothly nobody notices the grammar at all.
The director didn't set out to exploit anyone. She probably loves the story, probably cried making it, the grandmother probably cried too, Amir probably appreciated the attention. Everyone felt good.
Al Ghazali's work on causation versus correlation matters here. We see correlation (grandmother meets refugee, something beautiful happens) and mistake it for causation (grandmother's goodness caused the beautiful thing). But Amir's arrival in Berlin caused something too—his vulnerability caused something, his willingness to be filmed caused something.
When the person living the story becomes a character in someone else's story about themselves, that's not inspiration—that's a transfer of ownership.
”Nobody's lying. The grandmother probably was good to Amir, the director probably did admire her, the film probably is warm.