It also seems to factor into github search, OpenSearch is on the first page and ElasticSearch is nowhere to be found.
https://github.com/search?q=search&type=repositories
When I see that a project as been forked I use these stats to help select which fork to go with.
I guess ES has enough brand recognition. But it'd be killer for a lesser known project. While stars don't matter per se, they do give a project some credibility over a project with 0 stars or 50 stars.
If someone looked at both Elasticsearch and Opensearch and was new to the area, they'd think Opensearch was the original and ES the fork.
[I work for Elastic]
I'm the maintainer of a reasonably popular project (~9K stars) and it's certainly a nightmare scenario given that I consider the stars to give it credibility in a crowded space.