The computer and the program wouldn't exist without us, though. They only exist to be interpreted by us. The physical properties of the circuits outside of what we cajole them into doing are irrelevant, meaningless. The circuits only do their work regardless of particular interpretations; they wouldn't exist at all without people building them to be interpreted.
The physical computer could exist regardless of us. The program, if by that we mean "a human model of the computation happening in a physical computer" is just a description, yes.
It would be extraordinarily unlikely, but physically conceivable, that a physical system that is organized exactly like a microcontroller running an automatic door program, together with a solar panel, a basic engine, and a light sensor, could form randomly out of, say, a meteorite falling in a desert. If that did happen, the system would produce the same "door motor runs when person is near sensor" effect as the systems we build for this.
The physical circuit are doing what they are doing because of physics. They don't care why they happen to be organized the way they are - whether occurring by human design or through random chance.
Edit: I can add another metaphor. Consider buildings: clearly, buildings are artificial objects, described by architectural diagrams, which are purely human constructs, and couldn't be built without them. And yet, there exist naturally occurring formations that have the same properties as simple buildings - and you can draw architectural diagrams of those naturally occurring formations; and, assuming your diagrams are accurate, you can predict using them if the formations will resist an earthquake or collapse. Physical computers are no different from artificial buildings here, and the logic diagrams and computer programs are no different from the architectural diagrams: they are methods that help us build what we want, but they are still discovered properties of the physical world, not idealized objects of our own making; the fact that naturally occurring computers are very unlikely to form doesn't change this fact.
I disagree that it’s conceivable that a computer could somehow exist without a conscious maker. It’s so unlikely that it may as well be impossible. If something non-human that was capable of consciousness did form in the universe, through known biology or not, it would “just” be another form of life, and not what the paper is talking about.
What you say about buildings is sort of true as far as it goes, but irrelevant for the argument because buildings aren’t symbolic manipulation machines that only mean something via conscious interpretation, that some people are claiming could gain consciousness themselves.
Probability of such a structure forming is completely irrelevant. The argument makes sense if there was a mathematical/physical impossibility, but as long as the laws of physics allow such an object to exist and form by random chance, and predict it would operate exactly the same as the consciousness-designed one, I don't see any reason to discount it.
I also think the arguments against this are contradictory. On the one hand, we have an argument that says that computers only work because a consciousness built them to implement a particular computation. On the other hand, we're saying that the same physical computer doing the same physical thing can be interpreted to be implementing an infinite number of different computations. These two seem to point in different directions to me.
