It was one of her tweets, it's on wikipedia and was widely "reported" at the time. It came in the context of her late conversion to Islam, and the blowback it generated (imagine being a high-profile personality in a deeply $religion-following country, and publicly abiuring that to become $aVeryDifferentReligion...).
If it was in a tweet, then it was well after the damage was done by the system Morrissey is describing.
The "damage" might well have been done when she was put in a Magdalene at 15.
Morrissey's (quite interested) statements on how she was treated, and the fact that she wasn't the nicest person, are not mutually exclusive.