it's a horrible tragedy.
It sounds like the convergence of bad factors and bad timing. The driver, seeing a nice, well marked state road, with a clear path ahead, light traffic, and good weather, all ideal conditions for AP, engages AP and takes his eyes off the road for a few seconds. Unfortunately, the timing was horrible because a semi just happens to cross in front of him at that exact same moment. And that scenario of a semi truck crossing in from of you is one of the rare cases that AP cannot handle.
This will obviously be something that FSD will be able to handle. Once the front side cameras become active, FSD will be able to better track cross traffic and slow down preemptively before the vehicles cross in front of you.