They do, but as any large AWS customer you want to reserve a lot of capacity, so that it's there when you need it and also it's a lot cheaper than spot usage. So, scaling down below the reserved capacity doesn't save you much if anything.
The AWS-hosted part is not the CDN afaik, which will probably make up the vast majority of their infrastructure.
Speaking as an ISP: Netflix traffic comes from the Netflix ASN and peering session. Their globally distributed CDN/caching/video file storage servers are not on AWS to the best of my knowledge. Netflix traffic does not come from our peering with Amazon's ASN.
If you're talking about OpenConnect, then I think they are really geared to just be a CDN and not much else.
Since they host on AWS, wouldn't they just scale down during non-peak times and save money?