Two Possibilities
1. Forced Error
The FBI really do have clear evidence on the identity of the ShinyHunter gang and the proclamation was an attempt to try and force their targets in to making give-away mistakes - for example by provoking them in to breaking cover in a way that would make them easier to track.
2. Publicity Fluff
The Head of the FBI remains concerned for the safety of their position at the top of the organization and wanted to make a splashy announcement to serve as "evidence" that they were making progress on a strategically important case.
The interesting bit in the middle is to game out what the real members of ShinyHunters would do when they learned of the FBI's announcement. Which is to go to ground.
Now, if the FBI aim was to scare them in to going completely off-line ... well, that might work, at least for a time. But on the other hand, telling a wanted person that you are hot on their trail - when you don't already have them within a good, tight dragnet... that just seems a bit baffling to me. What did the FBI think these people would do? They doubtless know that if they're caught and extradited they will be given the usual "cop a plea or we'll throw the book at you" gameplay and that even if they do plead guilty they'll be looking at 10+ years in the poke.
So they will have absolutely gone to ground - and may already been in or making their way to a location in a country that doesn't have an extradition treaty with the US.
Yes, I'm being cynical... but this is Kash Patel's FBI. A sniff test says this is another case of "Ready, Fire!, Aim..."