If you read the actual report, it mentions that they did a canary step but its effectiveness was undermined.
> In this event, the canary step correctly identified that the new configuration was unsafe. Crucially however, a second software bug in the management software did not propagate the canary step’s conclusion back to the push process, and thus the push system concluded that the new configuration was valid and began its progressive rollout.
> In this event, the canary step correctly identified that the new configuration was unsafe. Crucially however, a second software bug in the management software did not propagate the canary step’s conclusion back to the push process, and thus the push system concluded that the new configuration was valid and began its progressive rollout.