Yeah, it wasn't specified in this report how the seized coins were sold. It was possible those were sold via the spot market. It was also possible the coins were sold via OTC. But I heard of auctions of seized coins in the past.
What's interesting, however, is that when the $216 million worth of seized coins were sold last year, it didn't cause a massive dump. Instead, just mere days after the selling, the price of Bitcoin went up more than 25%. And it even continued to rise right after. So, this planned liquidation of around $131 million worth of coins won't probably be a big deal this time.