We, the public, know a little bit more about the contents of the notebook than the article lets on. See this for example, about a different hearing. There are some entries I would describe as “pretty incriminating”, and it seems like the notebook would be an important source of motive and mens rea evidence for the prosecution.

And to underscore the seriousness of the backpack gun: it’s the same kind of gun that was reported as used at the scene. The prosecution hasn’t yet alleged that the backpack gun is the murder weapon, but it is a reasonable speculation that they will. And if they do allege that, it is also reasonable to speculate that they might introduce ballistic and tool mark evidence at trial to match the backpack gun to bullets from the victim’s body.

The prosecution has not yet revealed the existence of any other physical evidence. No prints or DNA, etc. So there is a possibility, in a hypothetical trial, that the backpack gun might be the only physical link between Mr. Mangione in Altoona, and Brian Thompson in Manhattan.

So it would be a big deal to the case if the backpack evidence is suppressed.