When I last discussed this with AWS, having them scale it for you meant that you actually had to contact them to pre-warm the ELB whenever you expected more traffic. It didn't work for our use cases where we didn't know when to expect more traffic.
Yup, that hasn't changed in our experience. If you're looking at bursty traffic beyond what AWS will pre-warm for you, then this likely won't work. We have fairly consistent loads throughout the day.