The VMS ink is NOT iron-gall

[oshfdk]

I have to go to the Univ to access that article.  But from what you quoted it seems to use broadband IR, with wavelengths from 900 to 1500 all together.  

Yes, and if it was absorbing half of this band it would be visible, I assume.

Another paper I quoted stated that low absorption of iron gall ink in IR makes the detection of the ink difficult:

However, the detection of iron-gall ink can be difficult, as it is not readily visualized by IR imaging due to its low absorption in the IR.

The third (publicly available) specifically talks about 840-1000nm light and demonstrates how one type of iron gall ink remains fully visible, while the other becomes faint (though still visible). The manuscript is from the XVI century, and in visible light both types of ink appear high contrast, well preserved and easily readable (as expected for the aged iron gall ink). You are not allowed to view links. Register or Login to view. (paragraph 14)

As it appears, the medieval iron gall ink is not an ISO standardized substance, it is a wide range of different preparations with different properties. If some of them are fully visible up to 1200nm, this doesn't necessarily apply to others.