They wanted to shift the narrative to shrapnel because then the angle of the bullet that hit Trump would be up in the air. If Trump was hit with shrapnel, then the bullet could have come from anywhere. But if Trump was hit with a bullet then a layman could chart its trajectory. And guess what? The bullet didn't come from that kid. That's why it's important for the DS to suggest shrapnel and it's important for Trump to maintain the narrative that he was hit by a bullet.