Some of the victims of the scam were US citizens. This gives the US Department of Justice a responsibility to investigate and to shut down the scammers if they can.
The investigators determined that these assets are the proceeds of the scam, and that seizing these assets will significantly impact the scammers ability to continue scamming. So the DoJ attorney goes to a Judge and lays out this logic and asks for a seizure order. Judge agrees, and it happens.
The article is specifically about the seizure of the cryptocurrency, but there is a lot more to it. Multiple governments worked together to raid the compounds (rescuing many people), and to interrupt the flow of slaves. Many people were arrested. There are international warrants for several people who were not captured. And an awful lot of their money is gone. Being on-the-run without money is a lot harder than being on-the-run with billions to spend.