They go into detail on their technical decision making, very cool:
Choosing Clojure6 and Datalog7 was a calculated risk–but one which paid off. The Datalog model offered the simplest and most ergonomic queries, had strengths in graphs and relationships, and the entity model could easily transform to more standard formats. In–memory Datomic was chosen on the server, allowing complicated relational rule–based queries to map over tens of thousands of claims, linking sources to the attributes of entities. Using Clojure allowed for rapid prototyping and fast feedback cycles. Together, this allowed constructing arbitrary graph traversal queries, without having to round–trip from the database to execute recursive code, allowing the nature of traversing the domain model to be separated from the specifics of the relationship. How entities relate, and how relations work through time, could be developed as separate concepts. Using Datalog queries, it was possible to get every subordinate, as well as the time ranges they held true for, with a simple query like (subordinate-of ?a ?b ?start-date ?end-date)
