Breaches are a one-way state change, like death. If customer data WAS affected, then it IS affected, until customers take action such as disclosure, resetting passwords/keys, etc. If customer data was affected, AWS would need to advise customers to take remedial steps. Since they have not issued such advice, is it safe to assume that they had mitigation in place for this attack?
It could also be the case that they have monitoring systems on the host machines that would have reliably detected this issue being exploited, even before this specific vulnerability was discovered (or perhaps better to say, given the age of the bug, before being discovered by someone that disclosed it to Xen).