Well the other route presumably has a significant throughput. If 1000 car/min are in traffic, and the side-road has a 200 car/min capacity, the first 20% of people to leave the main road will experience no additional traffic.

Now, let's say that the 200 car/min turns to a bottleneck of 100 car/min if it's overwhelmed 10 mins downstream of the turnoff. If google routes >200 car/min to the turnoff, there will eventually be a significant traffic jam: cars will pile up at the bottleneck, and even if waze stops routing to the turnoff at that point, the jam would still travel upwards like a pressure wave until it hits the turnoff.

I'd have a hard time believing it's possible for waze to know beforehand what those throughputs are, so some situations like this must occur.