They are lying by omission.
One might reasonably conclude that the killer left several journals and detailed project plans for assaults on several targets with specific names of people to be killed. Indeed, this has already been reported. Of course, those people who were named are not going to want to have their names disclosed. Neither do the schools want the public to be reminded of their status as targets.
There is a legitimate fear of copycats who might try to get a better score the next time.
However, the explanation sounds like a pretext for concealing the motivations of the killer and the identity of the people who encouraged and advised her with her project plans.
She does not fit "The Narrative" and her written materials very likely trace back to "Non-Profit Organizations, Confidential Informants, or employed Agents of the Federal government.
There probably is no "manifesto" as such, unless the Feds wrote one as a cover story.
I’m speaking from memory but I recall one description went: ‘journals, diaries and writings that constitute a manifesto’ ... words of that nature that seemed to imply it wasn’t a single writing but a collection.