Incompatible or not isn't the point. Not releasing is what it's about. And yes, they can. But that doesn't make it right.
There were contributions sent upstream for Elasticsearch bugfixes and enhancements. Some of those PRs are still open, for example [1].
A sampling of additional PRs can be found in this blog post [2].
Further contributions upstream to Apache Lucene have been growing over the years. The new Approximate Nearest Neighbor support in Elasticsearch 8.0 comes from work that was sponsored by Amazon in upstream Apache Lucene [3].
[1] https://github.com/elastic/elasticsearch/pull/64513
[2] https://aws.amazon.com/blogs/opensource/stepping-up-for-a-tr...