The "largest crypto seizure to date" has come six years after 120,000 BTC were stolen in a Bitfinex hack in 2016. The stolen Bitcoins were worth $60 million back then and represented almost one-sixth of the total trading volume at the time. This would be worth almost $4.5 billion today. However, the DoJ has only seized 94,000 BTC which are valued at $3.6 billion. According to the law enforcement agency, the two conspirators planned to launder these bitcoins. However, they don't claim that the two were the masterminds behind the original hack.
The couple allegedly used various techniques to launder the stolen bitcoin, including splitting transactions up into thousands of smaller transactions to avoid traceability, using darknet markets, and converting the stolen bitcoins into other types of crypto such as Monero. The statement also named darknet market AlphaBay as one such platform that was allegedly used by the couple.
Soon after the seizure was made, Bitfinex said in a statement that it would work with the DOJ to try and recover the seized bitcoin. It said that if it manages to recover the stolen bitcoins, it will repay investors in its UNUS SED LEO token, which was created to try and backstop a fiscal hole created after an unrelated payment processor used by Bitfinex was seized by authorities in three different countries.
The importance of the seizure can be gauged from the fact that it has garnered attention from prominent names in the crypto world like Changpeng Zhao who tweeted saying that it is much easier to track crypto than fiat.