I think it's because people conflate photographic printing with photography in general, which as Mr. Benson notes requires silver in the film.* The Physautotype avoids silver by way of extremely long exposure times, and I don't think Flandre would even be capable of sitting eight hours for a portrait, never mind willing.

*: Blueprints and carbon prints from non-photographic negatives don't involve silver.

I do love the way he talks about the Woodburytype. "Now this is like the first person who ate an oyster. Who ever decided to try it?"