The analogy is apt for several reasons. As admitted, a car engine cannot simply be "scaled up" to reach 800mph. This makes the argument that rocket technology is already available to the public invalid. It necessarily does not follow that because small rockets are possible and exist, that a large one weighing hundreds of tons could exceed the speeds necessary to get into space or achieve escape velocity.
the difference is that no "car" can go 800mph.
It's not an engine issue, rockets however can go that fast when scaled up.
the only "cars" that do get into the region of 800mph are not in fact cars, they are rockets and other aircraft with unpowered wheels bolted on. You might as well use the fact that you cant scale a car engine up to allow a car to fly as evidence that car engines can't power planes.
What if a company claimed to have invented a car which could go 800 mph, but the technology was conveniently proprietary or out of reach fro others to reproduce?
Moller International has been claiming to have a
working sky car for many years now, and is only waiting on a few more big investments before they can begin production.
If the technology cannot be reproduced by others freely, and is controlled, then the claims that such things have been built, or
can be built, are dubious.
SpaceX, Lockheed Martin, Boeing, and other rocket designers are all government contractors, operating under government regulation, with facilities on government bases, under direct supervision of government managers. There is no implicit trust of transparency or independence.
In fact, the government uses most of these contracting companies as temp agencies. While you might work for Lockheed Martin by name, the actual work you do for, say the Department of Defense, is done on DOD facilities and under the direct supervision of the government manager. There may be a small Lockheed Martin structure, where you report to your Lockheed Martin supervisor, who then reports to the government manager, but the environment is the same. The government calls the shots, not your contracting company.