1. Amazon wants to make private changes to the management layer for their cloud offering and not share those.
2. ES doesn't want that, so the 7.11+ license restricts it.
3. Amazon doesn't want to have to explain to their customers why their ES offering is stuck on v7.10, so they're changing the name of it.
4. Elastic was really hoping this wouldn't happen, but they overestimated the value of their brand and Amazon called their bluff.
So yeah, nominally OpenSearch is unrestricted, but realistically few other entities are in a position to make or benefit from the private modifications Amazon will be making. For us normal people, ES and OS are equivalent today, so it's more about how they're going to diverge over time in terms of fixes, features, whatever.