In Chernobyl season one episode two, we saw that when Valery Legasov and Boris Shcherbina were flying to the disaster zone by helicopter, Legasov was saying that there is graphite, but he saw only fume. How could he know that there was graphite on the roof?
We don't have the perspective of Legasov in the photo you show here, we are behind him looking ahead at the path of the helicopter, while he is more able to look down. Helicopters often have windows that extend quite low. It's possible he is able to look down at an angle on the roof of the building they are approaching.
Graphite is a very dark, almost black rock-like material. I think we are meant to presume from his statement, that he can see a significant amount of black colored debris on the roof of the building. Concrete is the other rock-like material he is likely to see, but that is relatively light colored.