I suppose the authors assumed the reader is familiar with the benefits of the serverless model.

When running functions as a service, you don't have to administer the OS, you just request the computation, provide an input event, and wait for the results to be computed.

If you use EC2 you need to handle the work submission, the scaling and registration of the workers, and the security and life-cycle of the OS yourself.

It seems to me they didn't get all the benefits of serverless, because they had to write thier own custom workload management code. But probably it would still be simpler than managing the workload across thousands of EC2 instances.