Alexa smart home skills, such as the one used here, need to be notified explicitly when a device is removed from your backend, otherwise they’ll just carry on as usual.

However, it looks to me that there were two levels of failure here. The first is not notifying Alexa of the device removal, but the second is some clearly broken authorisation code. When Alexa requests a video stream it includes an OAuth token issued when the user first set up the skill, which apparently Wyze weren’t checking was still valid for the camera in question before serving up a stream.