Hi, I am one of the authors of the project. Thanks!
One thing that became clear to us while working on this project is the significant difference in how incremental views (or stream processing) store and access data, compared to how PostgreSQL looks at and processes data (batch processing). So, it made a lot of sense for us to separate the technologies (i.e., have a transformation layer, and a "serving" layer), but still try to integrate the two as seamlessly as possible.
One thing that became clear to us while working on this project is the significant difference in how incremental views (or stream processing) store and access data, compared to how PostgreSQL looks at and processes data (batch processing). So, it made a lot of sense for us to separate the technologies (i.e., have a transformation layer, and a "serving" layer), but still try to integrate the two as seamlessly as possible.