I don't think AWS mitigates DDOS. If an app is under DDOS at AWS and was using an Elastic Load Balancer, the app would have to pay for the inbound bandwidth through the ELB and would have to pay for scale-up to respond to the increased traffic load.
It'd be much more cost effective to pay CloudFlare a fixed monthly fee and stop the DDOS attack before it even gets to your AWS resources.