There have been quite a few cityscapes and skylines in the Aerial screen saver video collection (Dubai, Los Angeles, London, New York, San Fransisco, Hong Kong), but I don’t recall Chicago being among them, nor do sites describing these official screen saver clips ever mention Chicago. A very few Aerial clips have been delisted (some because of 1080p instead of 4K), but no city has been missing due to that.

Possibly what you saw was not part of the official Aerial screen savers collection?
I turned on Apple TV one day and for about 10 seconds I saw …
The screen savers do not come on when you turn on the Apple TV box, so this may have been something else? The screen saver starts after a set amount of idle time (nothing streaming).