As you pointed out, reusing parts of old books or manuscripts was quite common. Evidently, there was quite a lot of it going on in England just after the dissolution of the monasteries by Henry VIII. Their libraries had a lot of books that were "unwanted" and there was also a lot of new stuff getting printed that needed binding. Parchment is a very sturdy material (consider that it's essentially a bag meant to hold in the internal organs of an animal that may weigh hundreds of kilos).
Anyway, I guess the novelty here is that they were able to read much of the older work without dismantling the Turducken book (one article I read used that term).
Was a little surprised to see that the researchers seemed to be holding the book with bare hands. Would have guessed those sorts of things are usually handled with gloves but maybe this was about pioneering the technique on something considered less valuable.