I'm not a lawyer either, but as I understand it, a trademark is violated if it's likely to confuse people into thinking the product/service is from the trademark holder when it actually isn't. If Amazon's CEO experienced such confusion himself, that does sound like a slam dunk to me.
FTA: When the service launched, imagine our surprise when the Amazon CTO tweeted that the service was released in collaboration with us. It was not. And over the years, we have heard repeatedly that this confusion persists. NOT OK.
It does seem tricky. On on hand, they want to stop AWS using "Elasticsearch" in a product name because it isn't in partnership with Elastic co., but on the other hand AWS's product really does contain Elasticsearch, which is why they are changing their license. If AWS had a product called "Elasticsearch Service" which didn't contain Elasticsearch, then it would be pretty clear cut as that would be very confusing, but a product called "Elasticsearch Service" that really does contain "Elasticsearch" seems pretty self-explanatory.
Does it really contain ElasticSearch? It is a fork right, so can you still call it ElasticSearch? I don't think you should be able to use the name in this case, and you definitely can't say you are "partnered" with a company when you most definitely aren't.
But what are they confused over? "Amazon RDS for SQL Server" seems no more and no less confusing to me than "Amazon Elasticsearch Service".
As a user, I don't care in the least about the business relationship behind the product. I care about whether Amazon RDS works like SQL Server and whether Amazon Elasticsearch Service works like Elasticsearch. What financial arrangements, if any, are behind the scenes are not a concern to users.
I think the original link and the CTO disagrees with what "colloboration" means.
From Amazon's perspective, if they contributed a single fix, or asked a single question of ElasticSearch on the issue tracker, then this is a product born from colloboration.
It's difficult to think anyone is going to think that Amazon ElasticSearch is by anyone other than Amazon.
(AWS had EC2 before Elastic’s trademark was registered.)