While it's possible this it he FBI flexing some muscle that they have a backdoor into bitcoin's hashing algorithm, what seems much more likely (to me) is:
There is a more sophisticated hacking group which created this particular ransomware package. They sell this ransomware package to less sophisticated criminals.
(https://www.theverge.com/2021/5/10/22428996/colonial-pipelin...)
Is it so hard to imagine a scenario where the more advanced creators of this ransomware kit gave instructions to their purchasers on things like private keys, and the end user simply ignored them?
Somebody ignoring a warning when installing a software, and that allowing the FBI to subpeona access to the server where it was running, and grab this private key, seems FAR more likely to me than the FBI having a backdoor into BTC, or this all being a cover spy novel plot, or anything like that.