Molecularity of Saponification Reaction
You should know that the molecularity is not relevant in your case and has no significance for multi-step reactions, it is just applicable to elementary reactions. The relevant parameter for multi-step reactions is the, reaction order wich is originally an experimental parameter  indirectly related to the molecularity of all elementary reactions that makeup the reaction mechanism.
So for saponification the proposed mechanism includes 3 steps, the last one of them is irreversible and controls the reaction. The order of the reaction is considered 2, the molecularity for the step that you have mentioned is also 2 but it is just one step in the mechanism.
Also, there are two types of mechanisms through which this reaction takes place: a base catalyzed mechanism (with simple base or alkoxides) and an acid catalyzed mechanism.