Don't the people who the software has been distributed to get to require the source code?
Yes. Code must be distributed with the software OR a written offer to provide the code must be distributed with the software.
Any person who has the software may then freely re-distribute it for a fee or for free. In this case, the written offer must still be honoured by the developer.The GPL renders it not illegal nor immoral to "leak" the software and every copy is legitimate. Hacking would still be a crime if it occurred but the copies would be legitimate.
My argument is: If Cellebrite have distributed software containing GPL'd code and not packaged the source or a written offer, then they are in breach of the GPL. If they have packaged the written offer, then it stands for anybody in possession of the software and if they don't honour a request then they are in breach of the GPL.
If these products are in breach of the GPL then it is likely that similar products from the same company are also in breach. This would allow a court to issue a 'motion to compel' to Cellebrite to produce the code for inspection. If these products are also found in breach, then they could be forced to stop distributing, face a financial penalty, and/or forced to release part or all of the code. It could also allow the public to get access to the software via a FOI request to the department using it. Tenuous, but within the realms of possibility and worth trying.