As I understand it, Amazon took ElasticSearch and monetized it, therefore competing directly with Elastic (the company that develops ElasticSearch). Elastic felt Amazon was taking advantage of Elastic's commitment to open source, and started developing new features under a proprietary license, but releasing them for free. The idea was to prevent their work from benefiting a competitor.
Basically in this blog post, Amazon is trying to make the case that they're a Good Tech Company and Elastic are the Bad Guys or something. Generally I'm all for nuance and "the truth is probably somewhere in the middle", but Elastic is the rare company that truly is committed to open source (at least at the current moment in history) and Amazon is, well, Amazon.