No, it isn't. An argument from authority would be me claiming that X is true because Penrose says so. The parent poster had said "I guess there are people who still believe the mind is some mystical thing not determined by biology." To which I responded with a data point: a highly respected physicist (indeed, an authority) who professes to be, essentially, a Cartesian dualist. I wasn't even claiming that X is true.
HNers do seem to love casually slinging claims of logical fallacies at each other.
Had you started with: "There are a handful (and I do mean a mere handful) of scientists, most of them with no background in either brain or computation science, who think ...." and then later mentioned Penrose, you'd be right.
But you chose to lead with his nominal status derived from his life as a talented and (rightfully) celebrated mathematician, thus attempting to improve the otherwise less than marginal status of the quantum-centric view of consciousness with his authority.
This is argument from authority.
In Penrose's case, while his position is mildly interesting, most other scientists and philosophers interested in consciousness and mind/brain questions completely reject it.