750k is not inconceivable. What is inconceivable is that a significant portion of those "coins" were highly liquid. I agree that if the numbers are that high, it is largely in part due to the use of Gox as a web wallet. The fact that Gox was an exchange is not evidence in and of itself that all coins held there were readily available on the market. Just take a look at the order books -- it puts things in perspective.
If only a smaller fraction of these coins were liquid (say, 100k), then the remaining 650k were for holding, presumably using Gox as a wallet.
I still can't imagine that 6% of all coins have been held at gox, at a time when everybody should have known for month what kind of company they are dealing with.
I dunno, speaking as someone who bought my few coins at about $5, and didn't bother to withdraw remaining loose change at Gox and other exchanges, then ignored bitcoin for a year, it wouldn't surprise me if many of Gox's customers bought some coins for fun/drugs years ago, and haven't really paid attention to how flaky Mt.Gox has been.
For that reason, I wouldn't expect a large proportion of Gox account-held-BTC owners to rebuy. They're not going to think "damn it, the $200 in coins I bought 2 years ago could have bought me a new car--now I'm going to chase after $500 BTC!" They'd probably think more along the lines of "easy come, easy go." That's certainly what I would have done if I hadn't withdrawn almost all my goxBTC in late January.