You should not be conducting unauthorized penetration tests against third party infrastructure providers without permission. They have processes and systems and usually just wants a heads up of what you plan to test and t the duration / timestamps.

Cuz otherwise you look like a threat actor.

That’s assuming your vendor was pentesting AWS systems. If you meant you hired a vendor to pentest your own systems on AWS, that’s of course a totally different matter.

I’m fairly certain you are supposed to contact any vendor before attempting to penetrate hosts with authorization, not the other way around.

Responding to an unknown security tester like that is a selling point, not a cautionary tale