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).
They took the closed XPack code and put it into the open source Elastic repo, but under a commercial license. It muddied the waters for anyone looking to use or contribute to the open source.
Prior to this move, the default install of the Elastic Stack was 100% open source. X-Pack had to be intentionally installed as a plugin.
Now the default install includes X-Pack, and you have to go out of your way (assuming you even realize that there is difference) to install the 100% Apache 2.0 licensed version.
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).