Flat roof, silver painted, so very likely a bitumenous waterproofing with a topcoat of bitumenous aluminium silver paint as solar protection. This is flammable, and is typically used on a concrete roof as waterproofing, as it is a very long lasting and easy to maintain layer, and is both cheap and fast to install using only simple tools and gas burners to soften the sheets and weld them together and to the concrete with a thin hot bitumen layer. It does burn if there is something that acts like a wick, and there is enough heat to melt the bitumen and cause it to wick up and vapourise, otherwise it is not too flammable.
Tthey know they are in a high fire danger area, and that the flat roof would definitely have glowing embers fall on it, and that they did not have a roof deluge system and tanks to keep the roof wet during this fire is just poor choice. Reminds me on watching CNN years ago, when there was yet another wildfire, and the aftermath showing the massive burnt residential districts, with only chimney stacks and roads showing above the ashes. In the middle of this was however a single house standing undamaged, built by an immigrant Korean engineer. CNN talking head interviewing him, and asked him why his house was the only one still there. His reply " I Thought everybody know wood burn", with his house, built from brick and plaster, with a moderately steep ceramic tile roof, with protected eaves and no wood exterior parts, that he built himself, as example.