I'm not sure exactly who Nyad is without more context, but since it's based on a true story, she's likely a real person who has had some kind of remarkable experience or journey.
I don't really know who Nathan Bartlett is in this context. He could be an ordinary person whose story is being told, or perhaps he's someone with a unique or remarkable story.
Definitely true. Diana Nyad's story is based on real events and her actual endeavors. Her perseverance and determination are inspiring and have been widely reported.