...but I think he is trying to say that if you have your limit order in for $30 and the stock is sitting at something lower than that...say $29...then...
First, there is no such thing as "the stock is sitting at...". Stocks don't "sit". The market has orders, which make "bids" and "asks". There are "bids" and "asks" sitting out there, not stocks. When you look at those bids and asks, you take away information, and reach conclusions, such as "the stock is selling at around this price". The market makes the price. But what if many (most!) of those bids and asks are fake. They are thrown in there in massive quantity by HFT and 90% of them are removed seconds (or even milliseconds) later. You are attempting to extract information from the bids and asks, under the PRESUMPTION that these are REAL bids and asks, that make a market.
If most of the bids and asks are fake, then where is the real market? You can't be sure. Therefore, the fakeout starts much earlier than the time you place your order. You place a limit order at 30 because of your observation of what you believe are other real people wanting to do real transactions. Perhaps you are wrong. If so, the HFT can skim you, and his work on you began even before you placed the trade.