[kronin]

Have you ever found confirmation that they use OpenDistro internally? I've looked and have been unable to find such a statement.

[ignoramous]

Though there's no confirmation I could find, there's an indication that they may/are:

Let’s take a quick look at the features that we are including in Open Distro for Elasticsearch. Some of these are currently available in Amazon Elasticsearch Service; others will become available in future updates.

https://aws.amazon.com/blogs/aws/new-open-distro-for-elastic...