OKcoin is not the only exchange... Yes, you can't trust many of the Chinese exchanges. But there are 10s of thousands of bitcoins being traded daily on many different (non-Chinese) exchanges that don't fake their volume.
it wouldn't be a flash crash if there was no demand. it would be a crash. the flash crash happened because there was insufficient liquidity on one exchange - as soon as the market sought to fill the gaps, it went right back up.
What happens if you try to cash in at these prices? There's no actual demand, so the price flash-crashes: http://qntra.net/2016/06/mere-265-bitcoin-sell-causes-flashc...