EBS snapshot resilience to AZ failures

If you want something to be used in compliance-related assessment you should use AWS Artifact. If your customer can download the SOC2 report and look at the Data Backup paragraph. You can clearly see 2 main statement:

Amazon S3 is designed to provide 99.999999999% durability and 99.99% availability of objects over a given year. Objects are redundantly stored on multiple devices across multiple facilities in an Amazon S3 region.

Amazon EBS replication is stored within the same availability zone, not across multiple zones, but customers can conduct regular snapshots to Amazon Simple Storage Service (S3) in order to provide long-term data durability.