Drearyweather's point is fine. If they just did this as a private act as most gifts are made, then I'd accept the argument.
What's different here is that they themselves made a public thing of it. They had someone take photographs and then publish it. If a missionary who nobody knows signed a Bible as a personal gift for a local official and had a picture taken would they publish it?
I think it's fairly obvious that the answer is no, because readers would be scratching their heads thinking - why did they include that?
So we can logically conclude that from their OWN point of view, they included it because Bibles being signed by the GB would in some way be a special thing beyond just a personal touch on a gift.