You're able to send voice messages to other people that have an Alexa. Basically like voicemail.

That means that, unlike butt-dialing, you're more likely to accidentally send a message to someone you're actually talking about since the person's name is part of the command. Yikes!

Is this something you have to actively set up or does it come enabled by default?

Yea, I wish the girl mentioned what the message started with. Something had to have been misinterpreted as a voice command and a contact name ... and the device UI has to be broken to the point where it doesn't give you any audio acknowledgement or confirmation.

Alexa has voice messaging built in to the device/Alexa app. You receive a notification in the app and can ask your device to play it.

Supposing that you're not tasked with resolving this problem for Amazon, why would those details matter?

So that we consumers can better understand this potentially huge vulnerability to determine if it's a bug, a "bug-feature", user error, user lying, or Amazon actually doing something evil.