ELB will actually do internal load balancing in a VPC using your own custom security groups. Doesn't help if you're not in a VPC, but nowadays the default is for everything to go in a VPC.
Unfortunately, Airbnb is a pretty legacy AWS client; when we started using it, VPC was a much less prominent feature. Migrating to a VPC is a very non-trivial process, especially if you have RDS instances outside the VPC
