I don’t see why that’s surprising. Technically-minded people, writing in technical contexts, value precision and unambiguity. Including diareses decreases ambiguity, and thus would be considered desireable in such a publication.
The word “cañon” is the Spanish spelling of “canyon.” The word originated in American Spanish, then was adopted into English. (From Merriam Webster: “Etymology: American Spanish cañón, probably alteration of obsolete Spanish callón, augmentative of calle street, from Latin callis footpath.”)
The age of the book does matter. When the word was first adopted into English, the Spanish spelling was used. As the word became more integrated into English, an alternative English spelling that preserved the sound of the Spanish original replaced the Spanish orthography. The original Spanish spelling is now obsolete in English, which is why one only sees it in older books.
Thank you for the comprehensive explanation. One thing that’s not clear, though, is whether you’re disagreeing with my statement that “cañon” is a perfectly valid (Mexican) Spanish word, meaning the same thing as the English word “canyon.”
In other words, are you adding information to what I said, or are you correcting me?
I’m aware it is the Spanish word, but I have always found that it is older books that use it. I’ve simply never seen that spelling in a contemporary work. Maybe it’s to do with the fact that Britain is not exactly known for its canyons, so English didn’t really need such a word until the United States had been fully explored. It was then borrowed and used as cañon until it had established itself enough as an English word for and English spelling to be used.
That’s pretty much what I figured, LoadedDog. That’s why Colibri’s response was a bit puzzling. It sounds like he’s saying that “cañon” isn’t a Spanish word.
No, I agree that cañon is a perfectly valid Spanish word, and in fact is the original form, from which the English word was derived. What I was taking issue with was the statement that the “age of the book doesn’t matter.” In books in English, a hundred years ago or so it was common to see the spelling “cañon,” whereas today that form is essentially never seen, having been supplanted entirely by “canyon.” This has a change in the way the word is spelled in English (or it could be viewed as changing from using the Spanish word itself, to using an English version of the word). If one does see the spelling “cañon” in a modern work, it is usually italicized, to indicate the Spanish word is being used.
Sorry if my original point was unclear.
Dude! You’re talking English spelling here! If the exceptions didn’t have exceptions, it’d be unAmerican! Hey, I here the Russians use a highly regular orthography. Anything you’re not telling us, comrade?
Fine by me 
Thanks, Colibri. I understand your point now. I didn’t realize that “cañon” was ever actually an English spelling.