My guess: as I wrote in another comment, Elasticsearch had significant evolution in the area of "search engine", while OpenSearch trajectory (to use the wording of the press release) has been in the direction of APM/monitoring.
When Elastic changed the license, all vendors of APM products based on Elasticsearch jumped to OpenSearch.
Maybe, they want to pass the message that the fork, OpenSearch, is enough different to not represent real competition, and in any case, now that it exists, after the investment done, AWS will offer that as an alternative to Elasticsearch and not Elasticsearch itself but managed by AWS as it used to be before the license change.
Now, who wants a managed Elasticsearch service will only buy it from Elastic.
When Elastic changed the license, all vendors of APM products based on Elasticsearch jumped to OpenSearch.
Maybe, they want to pass the message that the fork, OpenSearch, is enough different to not represent real competition, and in any case, now that it exists, after the investment done, AWS will offer that as an alternative to Elasticsearch and not Elasticsearch itself but managed by AWS as it used to be before the license change.
Now, who wants a managed Elasticsearch service will only buy it from Elastic.