We're calling this evidence, not proof, of the new system's radar x-ray vision because the video doesn't provide the whole picture. For instance, we don't know what caused the Autopilot system to issue the warning to the driver (it could have been a vehicle in a neighboring lane, say). It's also not known whether the driver intervened or the accident avoidance was all down to the car. It does seem like solid evidence, and the Twitter user who uploaded the video claims there was no intervention.
If this is indeed all legitimate, it shows what Autopilot can do in a situation where the driver might not have been able to react as swiftly – depending on the driver's point of view, he or she might not have been able to see the makings of an accident ahead.