They seemed to imply it was Geb by going straight to Geb after saying that it was a Berlin winner (he won 2006-9), but then ruled it out by saying still active. They show the guy's hands during the secret interview.
Here's a pretty good image of Bekele's hands. https://media.aws.iaaf.org/media/LargeL/356f5a0d-bb9b-4e32-a6c1-dc365bb2623e.jpg?v=822050358
The upshot: a bunch of the documentary was about a German agent Alexander (forget his last name), who seemed to be using and mistreating the foreign athletes he was bringing in. Some was about the many athletes transferring nationalities for money, 500 alone in 2010, I think they said, many to Turkey and Qatar, some to Azerbaijan. The head of Turkish athletics looks extremely dodgy, no surprise. The promises and treatment of these athletes was not always good. Then they shift to the national championships in Ethiopia, where despite Geb's claim that they conduct many tests, nobody tests positive this year, and the claim many were given at the champs, none of the athletes that they could see during the champs were going to doping testing. The tent they went to after the race just had their belongings. They then said the doping tests were in the stadium. They go to where it is labeled doping and nobody is there. They film through the window and it is empty.
Nothing there. The testing claim is a sham.
Then the interview with the Berlin winner. They show his hands. The voices are not supposed to be theirs, because they say they are protecting their identity and the guy says he'd be killed if he told about the deal. They spend a lot of time talking then about his agent (and Geb's), Jos Hermens who denies a connection to a German doping doctor (from Deutsche Telekom scandal in cycling) and the Freiburg lab, Lothar Heinrich. Yet, Heinrich was mentioned in an article about Haile's (supposed) last track race in 2004 and they found emails indicating that Jos was using Heinrich. What they don't mention is that Jos Hermens also had connections to a Spanish doping doctor as well. Healing Hans is also not mentioned.