An important detail not directly addressed in the article (or I missed it) is do the audio recordings include conversations between Alexa commands? Or where the audio recordings specifically related to the human issuing commands to the device? Either way, it's a huge issue the data was leaked to the wrong customer. I find it hard to believe this was a one-time mistake.
Audio recordings are of the wake word and following command. The device has lights to indicate when it's listening/recording.
Note that Alexa keeps listening after giving a response, so you can give a followup question without a wake word.