That's just the circulating supply of OPEN.ETH, not necessarily reflective of how much ETH was unfairly trapped when the bridge was removed. There's less than 30 OPEN.ETH for sale on the entire BitShares orderbook.
As far as I understand, the circulating supply is what physically exists in BitShares platform. If somebody luckily removed his open.BTS last year from Bitshares using OpenLedger bridge, the circulating supply would decrease.