Judging whether a fictional example of something is realistic is difficult because there are often extreme examples which can be quoted to support edge cases. That said, it's probably a good idea to look at the general type of Napoleonic battle.
The principal infantry weapon was the unrifled musket. In skilled hands, this had an effective range of about 100 yards. In practice on a smoke-covered battlefield, in the hands of conscripts, the engagement ranges were probably half that.
In contrast, the field artillery of the day was mainly smoothbore cannon with calibres between 6 and 12 pounds. This could fire solid shot with an effective (lethal) range of at least 1000 yards. In addtion to solid shot, they could also fire canister (essentially musketballs) and grapeshot (solid shot around 1lb), although these had shorter effective ranges.
The other key difference between the infantry and artillery, is that a musket ball will typically kill just one person. If it does completely penetrate its initial target, it has usually lost sufficient velocity to no longer be lethal.
However, a solid cannonball will have no problem passing straight through a man and killing the man behind (and the one behind that, etc.). Even a glancing blow will be enough to take a man out of the battle. There is a claimed example from the battle of Zorndorf (during the Seven Years' War) of a single cannonball, fired enfilade, cutting down over 40 men in the same rank. While this number should be treated with a little caution, it's safe to assume that a cannonball passing through a closely packed column of men could easily cause a dozen or so casualties.
So with the front lines engaged, any reserve units that are close enough to the battle to be of some use (i.e. close enough to react to plug a gap or reinforce an attack), are going to be well within the effective range of the field artillery of the day.
How many casualties this reserve might suffer is going to be open to a number of factors, such as;
- How many enemy guns are targetting them?
- What is the quality of those guns? (i.e. what is the chance of them hitting and how quickly can they fire?)
- How long is the reserve unit under fire?
- How is the reserve unit drawn up? (deeper ranks increase the chances of multiple casualties per hit)
As a fictional example, let's assume a couple of batteries of guns (i.e. 12 cannon) are targetting our reserves. These reserves are drawn up 6 ranks deep and are sufficiently numerous that 200 or so casualties will not reduce the chance of a hit. We'll be generous and assume that a single hit on the front rank disables all the men behind (so 6 men per hit). Lets also assume that the chances of a hit are 1 in 3.
So the first volley will cause 24 casualties (i.e. 12 * 1/3 * 6). So 10 such volleys will cause 240 casualties. Rates of fire for a gun battery were around 30 seconds per shot for the 6pdrs and 45 seconds per shot for the heavier 12 pdrs (assuming fresh, well-trained crews). So, with this example, our target of 200+ casualities could be achieved in 5-7.5 minutes depending on the calibre of the artillery.
Obviously, there are other factors that come into play in reality, such as morale - only a well drilled unit would stand and take significant casualties without breaking. However, the example from the book doesn't seem totally implausible.
Napoleonic Artillery, Dawson, Dawson & Summerfield (Crowood Press, 2007)