From the report's punchline:
The combined selling pressure from the Sell Algorithm, HFTs and other traders drove the price of the E-Mini down approximately 3% in just four minutes from the beginning of 2:41 p.m. through the end of 2:44 p.m. ... HFTs began to quickly buy and then resell contracts to each other – generating a “hot-potato” volume effect as the same positions were rapidly passed back and forth. Between 2:45:13 and 2:45:27, HFTs traded over 27,000 contracts, which accounted for about 49 percent of the total trading volume, while buying only about 200 additional contracts net.
HFTs don't like to hold stuff at the end of the day. Or hour, nay second. Further, HFTs that stayed in (some were obligated by their market-maker contracts with major exchanges to do so) helped the recovery back up.
One current big controversy among economists is whether the HFTs themselves have an effect on volatility (there are studies claiming all of "increases", "decreases", and "have little effect", but the evidence for any of those conclusions is weak, and it depends on exactly how you define volatility). A liquidity provider who provided added liquidity on very short time scales, but at the expense of increased volatility at longer time scales, wouldn't necessarily be advantageous for longer-term market participants. That'd be essentially trading one kind of noise for another one--- less millisecond variation, and more occasional giant aberrations, if indeed things like the May '10 "flash crash" were caused by HFTs.