They probably make use of multiple different currencies and precious metals.
As expected, it was a matter of time that media would start reporting on Bitcoin being used to finance the terrorist attacks. If it's true or not it's another story, I don't think they know for a fact, they are just speculating.
How is bitcoin an income source?? Do they some massive mining operation back in Syria or Iraq, or do they receive Bitcoin through extortion or something like that?
But I do think that the usage of crypto within IS is negligible, as if they're selling oil and receiving payment in bitcoin Slim chance....