A runner having a film crew running with them, even if they are running behind, gives a significant advantage that I believe should be illegal. I am not clear on the specific rules for the race, but whether or not it was illegal and should be grounds for disqualification, but it is unfair to the other runners that are struggling on their own.
Notably, one of the most impressive ultra endurance athletes, Lael Wilcox, ended up facing tremendous criticism and ultimately withdrew from the Tour Divide bike race due to the unfair advantage she received from having a film crew follow her. The Tour Divide is an unsupported race that goes from Canada to Mexico along the spine of the Rockies.
Like Diana Nyad, whether Killian Jornet cheated or not on his Everest climb will likely be impossible to determine. What is clear is that both Diana and Killian completely failed to follow commonly accepted standards for documentation. Significant skepticism is warranted, and neither of them should be able to claim that they completed the swim/climb.
In Killian"s case, he was sponsored by a GPS watch company, Suunto, but surprise, surprise on one of his hardest climbs that was the only time in years that Killian failed to have reasonable GPS data. Furthermore, it was established that Killian falsely used photos from his first climb in his video while claiming it was from the second climb.