Have you got a source or more information on this? I'm not disputing the statement - but I've seen several different languages claimed to run "most of AWS" (including Haskell at one point) and it'd be good to have a firmer answer.
I used to work for AWS. The API framework that powers most if not all of AWS API's is called "Coral" and is Java based.
It's like people think Java only supports concurrency via thread pool and isn't capable of event loop / async io models.