He was deceived by the false date of the letter, which was December 20th.
Jameson was intercepted by the Boers on New Year's Day, and on the next day he surrendered.

He had carried his copy of the letter along, and if his instructions required him--in case of emergency--to see that it fell into the hands of the Boers, he loyally carried them out.

Mrs.Hammond gives him a sharp rap for his supposed carelessness, and emphasizes her feeling about it with burning italics: "It was picked up on the battle-field in a leathern pouch, supposed to be Dr.Jameson's saddle-bag.
Why, in the name of all that is discreet and honorable, didn't he eat it!" She requires too much.

He was not in the service of the Reformers -- excepting ostensibly; he was in the service of Mr.Rhodes.

It was the only plain English document, undarkened by ciphers and mysteries, and responsibly signed and authenticated, which squarely implicated the Reformers in the raid, and it was not to Mr.Rhodes's interest that it should be eaten.