I'm not sure I understand Amazon's game theory on this one. Anything they contribute to their Apache License branch can be used by Elastic, but not vice versa.
This means Elastic can continue to differentiate from Amazon without worry. Amazon however can keep the code they've not contributed back internally, but they could always do that.
Perhaps this is simply an image move they don't plan to actually enhance or maintain the fork?
Also Elastic moving from Apache to (essentially) GPL is not making it closed source. Just as Amazon was within their rights to maintain an internal fork, so is Elastic to move to a GPL model. Demonizing either over business decisions is dumb.
It guarantees a future for open-source Elasticsearch. If there are contributors that want to use ES under an open-source license, now there is a steward for that code that will guarantee the code's long term support and open-sourceness.
And no, SSPL is nothing like the GPL. The purpose of GPL is to keep the code open-source. The purpose of SSPL is to others from using the software in certain ways (to make money), and its wording is not clear enough (and it does not have legal precedence) to make it clear what its actual limitations are.
Anyone may, and many people do, have open source forks of Elastic on github. 18.9k of them on github alone. There was no need for "another" other than PR.
I feel compelled to note that your summary of SSPL is wholly inaccurate. SSPL is very clearly a GPL derivative. It's language though leaves a lot to be desired, but the intent was clear. It does NOT prohibit making money offering the software as a service. It does require to contribute your changes back to the software if you are doing so.
The real failure is that companies have danced well enough around the edges of GPLv3 for so long that Amazon running a service which is basically just the software with modifications, but without the source code being contributed to open source, is feasible. This is even becoming a problem for Linux.
The reality is that Amazon Elasticsearch is clearly a derivative work under copyright law, and under GPL should be re-contributed. But major companies have "danced" the line so it does not have to be. SSPL is an attempt to bridge that gap, a poor one, but an attempt. The OSI appears more interested in defending their license than helping resolve this egregious situation. The SSPL sucks, but not having a solution for it sucks more.
This means Elastic can continue to differentiate from Amazon without worry. Amazon however can keep the code they've not contributed back internally, but they could always do that.
Perhaps this is simply an image move they don't plan to actually enhance or maintain the fork?
Also Elastic moving from Apache to (essentially) GPL is not making it closed source. Just as Amazon was within their rights to maintain an internal fork, so is Elastic to move to a GPL model. Demonizing either over business decisions is dumb.