This is interesting. Autopilot (without FSD Beta) will not go over 5mph of a road's known speed limit. Moreover, Autopilot will brake to a stop when it detects a potential obstacle and/or collision.

Riad hit and killed the couple after a left turn from exiting the freeway.

So either speed limit data was missing (in which case the limitation doesn't apply), he had his foot on the pedal and didn't know it (during which Autopilot will not brake; the driver is notified of this if the pedal is pressed down for more than a few seconds), or he turned Autopilot off before the crash (unlikely given the data that's in this article).