I don’t know that it really falls in to the category of having “doubts”. It reads more as “it’s possible that it was shrapnel” but they didn’t offer any evidence that it was anything but a bullet. Like what shrapnel may have come from. Maybe if they had shown where a bullet struck the stage and fragmented or something.The same FBI has doubts Trump was hit by a bullet...
FBI director suggests Donald Trump may not have been struck by bullet during assassination attempt at rally
Just read to me as “we can’t prove it was the bullet” with no evidence to support it was anything else.