Be glad it got corrected in only 20 seconds! I've seen a couple cases in another state with two minutes of dead air (until EAS timed out)!
I know of another case where the transmittter dumped in severe thunderstorm and chopped off EOM, until generator kicked in. The station got a good idea after flood of complaints and recorded a EOM on the automation to play in case the transmitter dumped the next time.
Sounds like a simple mistake of the SSA audio got routed instead of: "...this has been a required monthly test of the EAS system..."
Like I said, sounds like a simple routing mistake and it could have been far worse!