However, there's no quoted name here.

I think this is just a typo, as in {ggplot2}, we just put "diamonds" underneath the markup:

https://github.com/tidyverse/ggplot2/blob/6e8ac77790c90ef6f8cedf91f932bb77c7a4b19e/R/data.R#L1-L20