I can't tell if the author is describing Iron Gall ink (https://en.wikipedia.org/wiki/Iron_gall_ink) but it's not especially hard to make in small quantities if you have the ingredients.
This is indeed a recipe for iron gall ink. Different parts of trees were used in different locations, but gall nuts were common around the mediterranean, hence the name. It is, as you say, easy to make, but it can ruin a good fountain pen due to its corrosiveness.
