I'm not sure Amazon is willing to go all in to win the war of search services, for that means they need to to handsomely reward the best coders that contribute the most to the OpenSearch project to produce insane amount of high-quality code for better and new features. See the great article Code Hard Or Go Home: https://hypercritical.co/2013/04/12/code-hard-or-go-home. Nonetheless, there's a chance that Amazon may be determined to make OpenSearch catch up with ES, just like Apple has made Apple Maps comparable, even not better than, Google Maps. Therefore, open sourcing ES with AGPL is not a bad choice to retain the talent in the ES community.
And AGPL is kinda restrictive to many large customers too, as the customers do not want to risk being forced to open source their business-critical code. In fact, many companies simply ban the use of licensed software. Therefore, AGPL reflects quite the spirit of OSS while in the meantime will not undermine Elastic's business model.