You've been able to send video messages on Google Duo since March. The current implementation isn't particularly elegant, though; it's more like leaving a voicemail than intentionally sending a message. The app will receive an update this week that simplifies messaging, eliminating the need to call your recipient before sending.
To leave a video message on Duo now, you have to first call one of your contacts, then tap the Leave video message button that appears on the call screen. After the update, you'll swipe down on the home screen to bring up an interface for recording your message, then choose which contacts you want to send it to.
— Google (@Google) December 5, 2018
The old method will presumably hang around in case someone doesn't answer your call, but this new implementation should be smoother for when you explicitly want to send someone a video (and not have a conversation). Google says the new feature is rolling out this week, which is only three more days — so expect to see it soon.