Amazon has announced a new API for its voice services, i.e. Alexa. This enables one-way Alexa announcements within a household and from Apps without prompting by the user.
Although the Amazon blog did not comment about the use of Amazon Announcements concerning Emergency Alert System broadcasts (e.g. tornado warnings, evacuation alerts, wildfires, etc), this newe API may solve the previous constrain that Alexa devices only responded to user interaction. Previously, Alexa speakers could tell you there was a tornado warning when you asked for the current weather or passively show weather warnings on Amazon Echo Show screens. Alexa would not proactively announce warnings until you prompted it.
I don't know if Amazon will leave it up to 3rd parties to create Apps with emergency alert announcements.
Google Search passively shows some emergency alerts at the top of its search pages based on your location. Google Assistant devices do not appear to react to emergency alerts.
Netflix, Hulu and other streaming apps are likely the wrong layer for emergency alerts. Alerts should likely be part of the device management layer, to provide consistent behavior across apps instead of app by app.