Most likely they would be using Nginx. I find the offering interesting because most of the companies in need of a load balancer would be able to configure Nginx load balancer quite easily. Not knowing underlying technology and not being able to go deep into modifying the algorithms and other aspects of load balancer makes me slightly uncomfortable, so why should we go with it? The only positive aspect of this offering is automatic health check but Nginx does that already.
I think this offering is nothing more than a GUI for generating nginx configuration files.
Most people able to launch and configure Amazon AMIs could configure Nginx too. Yet Amazon offers Elastic Load Balancers. There's a huge market for stuff like this -- just because you can muddle your way through configuring an instance doesn't mean you want to be in charge of configuring and running all the moving parts.
It's a custom implementation. They were vague on giving any details, but if I recall correctly it was written in Python. So probably a Twisted, Gevent, or Eventlet implementation.
I think this offering is nothing more than a GUI for generating nginx configuration files.