Silk Road is not an exchange. It is a dark web market place that has been shot down about a decade ago by the American government, so that's the reason the recovered bitcoin could not be returned.
Which kind of begs a question - is stealing from an illegal site illegal itself? I don't think there's any identified victim in this case. If there were - bitcoins should've been returned to them. I don't think the "no victim - no crime" would be applicable here, but the whole situation is still kind of weird, especially considering he didn't hack the site, just took advantage of the glitch:
From what the news analysis was done, he got the bitcoins from the dark web and it was merely a back door to some sort of algorithm glitch. Whenever they bought any illegal stuff and clicked deposit the bitcoin, they received the double amount. Apparently, James was smart enough to find the loophole and he worked straight for 6 hours on the strategy to get as much Bitcoin as he could.
That's a huge amount of money getting traded at one time. It's a huge dump happening, so the price will certainly be affected. However, if that's really the main reason, then I guess the dump is just temporary since Coinbase has over $1.6 billion in 24-hour trading volume. Alternatively, the best idea is to divide the amount of Bitcoin to be traded in different exchanges, if the government allows it.
The article says they have sold the first batch and will be selling 4 more during the calendar year. So it's not like they're dumping it all at once.