Is this statement correct about writing and executing source mapping rules to populate identity attributes?

The statement is incorrect. When writing and executing source mapping rules to populate identity attributes, the rule itself does not need to persist the identity attribute to the database. In SailPoint IdentityIQ, once the value for an identity attribute has been calculated by a source mapping rule, IdentityIQ automatically handles the persistence of this attribute to the database as part of the aggregation or provisioning process. The developer does not need to explicitly persist the attribute; this is managed by the system.

SailPoint IdentityIQ Source Mapping Guide

SailPoint IdentityIQ Administration Guide (Aggregation and Attribute Mapping Sections)