As long as there's any American ownership in the chain, this is not to be trusted.

I'm assuming AWS wouldn't fully divest from this European business unit and split it off as a completely separate entity?

The US CLOUD Act says that if Amazon has the technical ability to access those machines, they must do so if the US government asks them to.

So, unless it’s a separate legal entity, and also shares no authentication, software deployment, or related infrastructure with the US part of Amazon, it’s either not providing sovereignty or is being offered in violation of US law.

It’s unclear to me if they’d have to comply with requests to (for example) backdoor their IAM service backend and push the binaries to Europe, or not. (I’m not a lawyer.)