Adding another thought to this, which is that I doubt the code behind this is a direct response to Elastic's recent license changes. This is a ton of work and I'm guessing AWS has been working on the code side of this since well before last summer. By all accounts their hosted Elastic offering is wildly successful so they've probably been hard at work to add features that Elastic formerly had as closed and commercial (before the license change).

I think their open sourcing of this work is a direct response to the recent license changes. If it weren't for those, they might have just kept this work closed and in use only for the hosted product (like they do with many other projects).