The extra deck armour is a game-balancing device rather than a historical feature of Dante Alighieri. With the fire-control and gun technology being modelled, long-range plunging hits are less likely than in later eras, so the artificial deck weight has less effect on the test. Increasing the belt instead would change the ship's resistance in the range band the battle actually uses and create armour implausible for the period. This caveat keeps the simulation's stability workaround separate from the historical design argument.
Notable Point · 1:05:42-1:06:31
Why the simulation adds deck armour, not belt armour
The reconstruction uses extra deck armour as a balancing workaround because early fire is less likely to arrive at steep plunging angles.