This is not true at all. The Skylines traffic model is agent driven. The residences have residents who travel to shopping, and the industries have to receive raw materials and ship finished goods to retail or export. Despawning does occur, but if there is too much despawning and the industries cannot get their raw materials or deliver their goods, they will complain and eventually close. Same if the commercial zones can't receive deliveries or get customers. Garbage trucks need to be able to complete their rounds or garbage piles up, and dead bodies need to be picked up. There are map views that show exactly what trips are using what roads, and you can trace a vehicle from source to destination.

Despawning can be thought of as what happens in any city when the traffic grid is overwhelmed: some travel simply does not occur at all. People stay home or they do not transact business because traffic is too bad.

It's not a perfect traffic model - for instance, residents don't really need to get to work or school; they are just teleported there if there's no route. Cars do not need to be parked. So a city sim with the Factorio devs' obsession with optimization would be great. But it's not accurate to say that Skylines traffic is just derived from the statistics of the city.