Yes, they can. However, that will leave their trails in their KMS service CloudTrail - unless they manage to exploit CloudTrail as well. That's a lot of barrier to bypass, especially because accessing all these services require you to be in the correct permission group with a hardware MFA token.
Somebody can access the key hardware but they can't extract the actual key out of that. However, I've never met anyone with that level of access - and AFAIK you have to go through various security clearance and approval before such human intervention is permitted.
There's no such thing as perfect security - but KMS is as solid as I can see with centralized key management at the moment. And customer can roll out their own key server as well that is managed in your own data center.
Somebody can access the key hardware but they can't extract the actual key out of that. However, I've never met anyone with that level of access - and AFAIK you have to go through various security clearance and approval before such human intervention is permitted.
There's no such thing as perfect security - but KMS is as solid as I can see with centralized key management at the moment. And customer can roll out their own key server as well that is managed in your own data center.