If you look at her face it isn’t quite as well done as the body. And the racing number is a completely different style than all the other runners in the race. So my hypothesis is he found some other photo of runners taken from a similar angle, copied a body from that photo to this photo (except the right arm), and then hand edited the face/edges to make it match.

I have no idea, but it's impressive. And yeah, this guy has been doing this for years before AI.

Going with the 'not AI' assumption, if there were more pictures he might have been able to use one where she was more visible.