There's a nerd law idea where everything needs to be 100% scientifically proven and reproducible for a criminal conviction. That's not how the law works at all. The law works probabilistically, and when the doubts are low enough than it's goto jail, even for criminal cases.
The prosecution would present the Cellebrite 'evidence' to the judge and (in our scenario) it would move the needle towards the prosecution. Then the defence will (always) try to doubt the evidence and move the needle back, but it would take a lot more than 'something might have happened, dunno how' to do that fully. When the needle is tilted enough, the prosecution has their conviction. Note that in our reality, judges have a lot of trust in prosecutors, and the needle always starts tilted...
"Nerd law idea" or not, this should bother us. Some "expert" saying some software found incriminating evidence, but, oops, you'll just have to trust us that it works as we're telling you it does... no, no, no, that's not justice at all.
Are you suggesting they can't hack phones, and just fake entire address book/location history/all messages? That's obviously false. Is the suggestion that there was some sort of massive conspiracy planting evidence*? I strongly suspect judges won't assign much credence to that.
* Such a conspiracy would have to include Cellebrite, the police and Apple/Google. After all, hacking the phone likely means you have the iCloud keys and can download the backups to verify. Furthermore, the conspiracy would always be at risk from the accused choosing to restore the backups to show some evidence wasn't there. So someone at Apple/Google would have to be complicit as well to modify the backups.
Science also works probabilistically, like most of it, but the law works "probabilistically", as in eyeballing a rough estimate using folk theories kind of probability.
The prosecution would present the Cellebrite 'evidence' to the judge and (in our scenario) it would move the needle towards the prosecution. Then the defence will (always) try to doubt the evidence and move the needle back, but it would take a lot more than 'something might have happened, dunno how' to do that fully. When the needle is tilted enough, the prosecution has their conviction. Note that in our reality, judges have a lot of trust in prosecutors, and the needle always starts tilted...