Yeah, to be honest this seems suspicious. Either it is inaccurate reporting, or there is something potentially fishy going on. List of possibilities in no particular order:
* The "hack" was all done by the FBI as a false flag operation
* The FBI has access to encryption backdoors used in some blockchains
* The DarkSide group re-used previously cracked keys with their wallet (I'm not sure this is even possible with any crypto, but throwing it out there as a maybe)
* The FBI hacked into computers owned by DarkSide and obtained the private key from them
* The FBI controls more than 51% of hashing power for the crypto used here, and transferred the coins to their own wallet using a 51% attack
* The "hack" was all done by the FBI as a false flag operation
* The FBI has access to encryption backdoors used in some blockchains
* The DarkSide group re-used previously cracked keys with their wallet (I'm not sure this is even possible with any crypto, but throwing it out there as a maybe)
* The FBI hacked into computers owned by DarkSide and obtained the private key from them
* The FBI controls more than 51% of hashing power for the crypto used here, and transferred the coins to their own wallet using a 51% attack