After looking at the so-called code for a while, Gibbs realized he was seeing a common form of medieval Latin abbreviations, often used in medical treatises about herbs. ... So this wasn't a code at all; it was just shorthand. The text would have been very familiar to anyone at the time who was interested in medicine.
Yay for solving the mystery and all, but it's such a mundane solution that I can't help feel a little sad. Update: via MacDara Conroy on Twitter - Has a Mysterious Medieval Code Really Been Solved? (I'll save you a click: no)