Earlier quoted context omitted.
That supplier may be in violation of their contract. If Nord put in that there are to be no undisclosed methods to access the supplier system they're renting, and there are, this doesn't change any facts about the incident here. If I was a Nord user, I wouldn't care that the supplier will refund Nord their service charges. I don't think "no one could know" is ridiculous on it's own. Think about the level of access yo…
AWS has external auditors verify their policies, procedures, and actual methods meet a wide variety of compliance requirements from many different agencies. The level of access those auditors and other verification methods have to AWS is not none but very significant. https://aws.amazon.com/compliance/programs/
Now on topic... You could argue that Nord perhaps was a bigger client than you or I am to AWS, and maybe they should have had better access, but the fact of the matter here is that it's absolutely possible that Nord is being accurate when they say "[we] could not have known".
Contract violation or not, you should never have full 100% confidence in someone else's system. If I was Nord and renting cloud I would absolutely assume there were undisclosed accesses, as I bet they are viewing everything now.