I drive AP at full attention to the road. And should have been more wary of this truck, for sure.To your point, I don't think we would have made it out by punching it once the truck started to move over. When the truck began to encroach my foot moved to the brake, which already engaged fully engaged - assuming by AP, maybe just AEB? We went from about 75 to 30mph in less than a second. One hand went to the horn, and the other to try to split the difference between the concrete and the truck. AP dialed it so quick and so precisely, I didn't have anything better to add to the control. That's my memory. I wish I had been more cautious around this truck. I assumed we were within his sight lines because we were overtaking from 100+ ft behind. Did not see the useless mirrors on the truck, or lack of signaling. I did not appreciate this driver's stupidity.