Captain Tauber was an experienced artilleryman and a battery commander at the time he wrote the book. Much can be learned from being a school-trained artilleryman and using the issue ammunition. Training and experience are everything. Further, he was trained in and also experienced in gunnery and ballistics if the curriculum of the Prussian artillery school was up to par, which by the time this book was written it undoubtedly was. The Prussians learned their artillery lessons from the Napoleonic period and their artillery arm became excellent in both skill and equipment before the next European go-round when Prussia launched on the Wars of Unification.
What is definite is that by the time the canister round left the gun tube, the individual balls were free of the canister. That can be proven by the gunnery tests when canister was fired at a fixed target. Tin isn't the strongest of metals and the act of firing the piece with an adequate charge would be enough to rupture the canister and get the desired effect of the round.