Plus the task isn’t to accurately mimic the properties and look of fresh, historic ink. It’s to mimic ink after it’s been on the page for centuries.
Just using accurate iron gall isn’t going to look like an old document. It sounds like they used carbon as the black pigment, probably soot. Which was a common enough base for inks, paints and what have historically in the form of lampblack. Then it’s mostly the titanium white and gelatin to bind.
Seems like a decent mix for a faded look.
The entire surface was also treated/coated with something, leaving a residue of glycerol monostearate. Apparently a soap derivative used as an emulsifier, thickener, and in cosmetics.
That seems a pretty common approach in forgeries, even where original paper is used. Odd ink mixtures and after treatments or altered surfaces to mimic age.
Today’s forgers have a bit the same problem. In that period accurate materials won’t look old if simply applied as is or as they were at the time. Even to actual period materials. A host of alterations, treatments, coatings and what have have to be worked in to artificially age things.
Old iron gall ink for example isn’t black, even if it was when it was laid down. It tends to, basically, rust to a sort of dusty brown. It’s also quite acidic, so it damages the paper/parchment in a rather tell tale way.