Additivity for the comparison map #
The two constructions flanking the comparison map of Deligne's (2.11.1) are additive: the tensor product of super modules is additive in each variable, and realization turns a finite sum of endomorphisms of a module object summing to the identity into a finite sum of endomorphisms of its realization summing to the identity. These are what let a decomposition of a module object into a finite family of retracts be pushed through the comparison.
The tensor product of super modules kills the zero morphism in the left variable.
The tensor product of super modules takes a finite sum in the left variable to a finite sum.
Additivity of realization #
Realization is additive on endomorphisms: a finite family of endomorphisms of a module object whose underlying morphisms sum to the identity realizes to a family summing to the identity.