Yep. That’s right folks! Forget everything you thought you knew! Its all supposed to be in C##, but the publishing company’s wanted to save money on ink. 
But of course there’s allot more to the story. Of course the two evolved together, music and instruments influencing each other.
Well, it’s simpler than that, of course, if you’re willing to put the horse in front of the cart. D whistle is the concert pitch whistle, that’s all. And since whistles in other keys are treated as transposing instruments, the D fingerings are universal, and a D tutorial can be used with whistles in any key.
And welcome to the Chiffboard. 
Just to clear up a few misconceptions you may have, perfectly normal when coming at ITM from a formal art music background.
Well, rote implies memorization and imitation. Those play a part, but the creative process of tune transmission is more complex and interesting, and not just past tense. And it’s hard to generalize about “traditional music around the world”, when different traditions may vary widely in regard to the role of theory, formal instruction, media, notation, formulaic repertoire, etc.
From the point of view of folk communities, it’s not at all clear that that’s an advantage. In fact, possibly just the opposite. For example, making it easier for the world to steal and mishandle your musical heritage without participating in your folk process, and without giving back to the community to which that heritage belongs, is not something that is necessarily smiled upon, except by the people interested in doing the stealing …
You’ve never been to a session, have you? 
Showing up and sight-reading tunes is a pretty good way to guarantee you’ll be shown the door. In fact, sight-reading tunes you’ve never seen or heard before is pretty much exactly the opposite of what sessions are about.
You’ll find that making a distinction between songs as wordy things that are sung, and tunes as things that are played, will serve you well. Calling tunes songs makes some people crazy, though they’re probably already crazy in the first place.
Well, think of it this way. If all the notated music in the world disappeared tomorrow, would you be able to continue to participate in ITM at basically the same level? If not, yer doing it wrong. That’s not so say the contrapositive, that good reading skills prevent you from being a good trad musician. But the essence of the music is overwhelmingly in the non-written aspects. And if you rely on notation as more than just a convenience, you’re missing most of it.
In any case, welcome to the music and to the whistle. If you stick with it, one day you’ll look back on your comments and chuckle. But the process of learning to rethink things is part of the fun.