The fireman's observation was that the plane was already on fire before it reached the piece of debris. This could be determined easily by watching the full video of the plane's takeoff, which I presume was available to the investigators. I haven't seen a comment or picture either way from the released material; only the reconstruction cartoon.

The BBC carried an item that the gas tanks were insuficiently insulated from debris entering the wheel well.

As I said, only the first shot has been fired, and there will be much more to come. I don't know who was most responsible; none of us commenting here are able to determine that.