I think that those facts about "correct" singing techniques are more true in the world of classical music & such. in pop the artists don't really care about whats the "pure" way of singing , they care more about the singing actually sounds.
like Mariah Carey has her own way of singing , the technique she uses is her own & not a "pure" singing technique. she has her own unique sound which soundfs really good , tho ppl say that not using the correct techniques has lead to some deteriorating in her vocal abilities over the years
anyways , my point is that it's clear that Ayu's vibrato/warble/worble/whatever isn't a "true" singing technique but really it doesn't matter to the casual listener in pop music
