He's saying that due to correlation and that the price movement begets price movement, framing the market in terms of normal (random statistical) distributions is not necessarily accurate. In statistical (and mathematical) terms, this is called regression (where the previous output is a subsequent input).
This was the basis behind FiveThirtyEight's presidential poll modelling showing 2-3x the chance in prevailing narratives of Trump winning. They explained at length (seemingly daily) that errors within their model were correlated such that under reporting in PA would mirror under reporting in FL....also, Nate Silver is a f'ing genius and a hero of mine.
So, to the extent that a stock that performed poorly in the past will continue to perform poorly, and a previously over performing stock would continue, random statistical samplings are not accurate. These are the so-called "fat tail" distributions that form the basis of long OTM option strategies...and likewise, a poor performing stock in a bull market will perform well, as will an over performing stock perform poorly in a bear market.
I suspect the majority of profitable option strategies are premised on this. Certainly profitable long strategies (false leverage strategies notwithstanding). As a premium seller, this directly influences my 'soft stops' which are subjective exits earlier than the hard stop on a stock likely to continue its decline (or advance).
Another couple other good things to point out, certain prices tend to trade more frequently (150.00 will always show more ticks than 150.47). $0 is a floor price, so the entire tail of the normal curve past there is inaccurate. A "t-Distribution" is another good one to know. Because companies maintain intrinsic value (read some Buffet shareholder letters), you won't have a true normal distribution. An undervalued company will supposedly have more people looking to buy, such that the curve's peak will still be at the current price, but substantially more than half of the area under the curve will be above the current price.