This event has no closures and has a polygon defined, but is displayed as if it were in the entire city, both in client and in the web. Is this WAD when an event has no closures?
Great example! Yes, it can be WAD, but it depends what the intended result was.
Submitting a polygon event with no closures is really helpful for traffic events that involve a lot of traffic disruption but no actual closures. It allows Waze to still send a push notification to impacted drivers and display a notice in the trip overview screen when drivers are getting a route.
I hope this helps 
Wouldn’t it be better if the map showed the perimeter of the affected area, just as it does for events that have closures?
Probably yes. I’m not sure who created the event, but you can speak to them about it. If it’s in your area, I think you can also adjust the polygon yourself - I can’t remember if it will automatically update the event page or not. Try and see