Would you be willing to publish the contract with sensitive information redacted?
All these bold claims are just claims until people come up with some substance. Talk is cheap and confirmation bias happens all the time.
The point still remains: let's see what the LLM delivered that the user actually used. Either it's legally binding and an appropriate use, or it's not fit-for-purpose.
Equally, why not an interactive form using conditional logic? No hallucination possible. Much more simple and reliable.
If you know this, you don't need GPT.
If you don't know this, you don't have a way to assess GPT's attempts at a contract. A bill of sale is indeed simple, but there's a lot of more subtle legal issues someone might run into in life.