They also probably have one or more test regions where they could perform a test like this. But it's presumably not at nearly the same scale as us-east-1, the region affected by this incident. And to a considerable extent the problem was one of scale. The writeup makes the recovery sound fairly straightforward; but due to the sheer size of S3 in this region, it took hours for the system to come back up, which was apparently unexpected.

(Nit: this incident affected a region, not a zone. us-east-1 is a region, which is divided into zones us-east-1a, us-east-1b, etc. S3 operates on regions.)