Actually, I am not sure if anyone actually got the translation of the Aya's interview on Letty right.
From what I read, it seems like Letty does not want others to think that her ability is the same of Cirno. It is never mentioned that she hates Cirno or something.
Think about it. If it is canon that Letty hates Cirno, how come there isn't any fanon that shows her beating Cirno senseless?
There probably is, somewhere - but that's just a guess based on the size of the Touhou fandom. I'd agree that Letty's interview just suggests that she wants to make it clear that she and Cirno have different (but similar) abilities; mostly it seems to me like she (as a "proper youkai") doesn't want people to consider her on the level of a mere fairy.
The "Shot out of canon" pool is a bad idea as currently described (it's way too wide-ranging), and Touhou is a particularly bad place to apply it, since ZUN quite specifically avoids going into much detail about the world.
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