I would guess that this specific advice would be for those who already own the securities currently being sold at 50% "discount".
But regardless, when you have a derivatively priced security like RSP (which holds actual stocks -- it has some inherent worth at all times that's knowable) any selling at a discount is supposed to be arbitraged away. In these cases, it wasn't,because the trading windows were too short.
(Hi, I'm the author of that blog post).
I assume you are referring to naked short selling.
To be clear, anyone may borrow shares and then sell them. Although legal ownership does pass to the borrower, this is not what most people think of if told they must "own shares prior to selling them".
In fact under SEC regulations, the borrow doesn't actually need to take place before the sale, so long as the broker has reasonable grounds to believe that the security can be borrowed to satisfy delivery for the sale.
From the retail traders perspective, this is typically transparent. They simply enter a sell order in a security that they don't own, and their broker worries about locating it for delivery (or doesn't allow the trade if it believes it can't locate the shares).
Until 10AM that day, the problem was zero buy-pressure. No bids at all to speak of.
http://www.etf.com/sites/default/files/images/2_rspsurveyor....
Also most high-frequency traders are market-makers or unofficially acting in that role. ISO orders are used by all executing brokers trying to sweep the best price, it's equally likely that the low sells were by funds executing stop loss orders through a brokerage or bank or wholesale market makers offsetting inventory at a loss after providing liquidity to retail stops.