I was looking at the main AMI list when you launch an EC2 instances. I am not sure if they are actually created and maintained by AWS, but at leas they show up on the main page makes me think they are "official."
10.3-RELEASE is a new version, yes. (It was released last week, but it took a week to get the new version into the AWS Marketplace. I'm still working out the kinks in the process.)
The "since 06/06/2014" is for FreeBSD 10; there's a separate Marketplace listing for FreeBSD 9, which shows "Available on AWS Marketplace Since: 11/20/2012": https://aws.amazon.com/marketplace/pp/B00AA25MLK
Before you had to check a list of amis on Percival site's select the right one at your region. So it was not so friendly.
Thanks to Percival for this! It is a huge step in making FreeBSD more friendly for beginners.