I left AWS before Aurora was introduced, but I suspect it's similar to how S3 fulfills a similar promise. From the S3 FAQ[1]:
> Amazon S3 Standard, S3 Standard-Infrequent Access, and Amazon Glacier storage classes replicate data across a minimum of three AZs to protect against the loss of one entire AZ. This remains true in Regions where fewer than three AZs are publicly available.
> Amazon S3 Standard, S3 Standard-Infrequent Access, and Amazon Glacier storage classes replicate data across a minimum of three AZs to protect against the loss of one entire AZ. This remains true in Regions where fewer than three AZs are publicly available.
Italic emphasis is mine.
[1] https://aws.amazon.com/s3/faqs/