Penrose believes that consciousness originates from quantum mechanics and the collapse of the wavefunction. Obviously you couldn't (effectively) simulate that with a classical computer. It's a very unconventional position, but it's not circular.
This is just another form of "god of the gaps" - Penrose desperately wants an interpretation that would allow for freedom of will, and so he constructs a theory around a physical process that allows for randomness, despite there being no evidence that this process is actually relevant to consciousness.
https://en.wikipedia.org/wiki/The_Emperor%27s_New_Mind
https://en.wikipedia.org/wiki/Shadows_of_the_Mind