An autopilot is a system used to control the path of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator's control of the vehicle, allowing the operator to focus on broader aspects of operations (for example, monitoring the trajectory, weather and on-board systems).
That's just devious marketing on Tesla's part. They can always excuse customer misunderstandings with the original meaning you explained, while normal people can be savely expected to interpret autopilot as full self driving (and I'd be surprised if they didn't have actually tested this with focus groups beforehand). So not really lying (great for the lawsuits), but constructing misunderstanding on purpose (great for the brand image).
