“All a website would need to do is embed the above in their website and any Zoom user will be instantly connected with their video running. This is still true today!”
Guess I was a bit naive in thinking just trashing the .app and immediate artifacts in Library would do the trick.
EDIT: I missed the .zoomus directory in my home folder that had the culprit. Funny enough Zoom's instructions on how to uninstall the app on macOS just points to documentation from Apple and wikiHow (???) with standard methods that don't fully remove Zoom.
Additionally, when you launch the main application, it will check to see whether ZoomOpener is running. If not it will boot it up. The main app will install and register ZoomOpener as a Login Item if necessary.