As for Chinese papers, this is speculation territory but with some personal knowledge. You can buy Chinese identities, there's a market for it. I know people who previously revoked their PRC citizenship but later acquired a dead persons identity to buy property and start business in mainland among other perks.
This is the crux of the problem. We have no idea whether they are legitimately being investigated, or if they are innocent people being held in the country to apply pressure on someone else.
A low-level employee who travels to the PRC for work might end up in the same situation if his company faces accusations, and anyone who travels to the PRC, or an HK citizen who might face extradition for trial in the near future, needs to be aware of this.
I mean, the arc of your claim (that this is just fallout of an otherwise legitimate crime investigation and not a deliberate punishment for these kids or the US in particualr) is probably true. But still, fabricating arguments against them doesn't make it OK for them to be held against international law.