Sorry, to say that they are sticking to their guns is quite an overstatement here. I would agree with you if in their response they share the results of their internal testing and say that it is matched by the testing done by a respectable publication. They did not do that. Under these circumstances I would conclude that either they do not have the data, or their internal data do not quite match the Autocar data.
Yes, this is something I am fairly certain about. Unfortunately for your case, there is a documented road test, from another US based respected car magazine,
Car & Driver that
uses rollout in their testing, indicating that McLaren F1 0 to 60mph time is 3.2s:
So the picture is quite clear now: Tesla had a legitimate reason to make claim about matching McLaren F1 performance, and that claim is a reason in itself that they switched to using rollout for P85D.