It's also corroborated by people who've actually been trained at Arsenal.
Worse players make more mistakes. But when the same players make the same mistakes, over and over and over again, and the sorts of mistakes that hardly ever get made at much lesser clubs by much 'lesser defenders', and despite promising to do so the manager seems powerless to actually halt the slide, then it's pretty obvious it's coaching.
As for the difference between attack and defence - not really strange at all. Completely different ways of playing.