Sometimes a better approach is to wrap the system you want to deprecate rather than have a parallel implementation. First, because you might not fully comprehend the current implementation to replicate it. Second, because in real life often there's not enough time, budget and coordination between teams to migrate old clients.

Since their main requirement was having a REST API for new clients, and apparently the old implementation worked fine, they could have implemented a EJB -> HTTP layer on top, and have new features talk straight to the DB. Eventually, they replicate all the features talking straight to the DB (w/ the added benefit they can test against the codepath going thru the old system). Finally, death by phagocytosis.