This is classic Business Insider. They explicitly ignore the context and try to shake you up with a cherry picked quote. Jamie Dimon doesn't think, and doesn't want his reader to think, that October 15th was a once in a 3B year move. Specifically, in the letter, Jamie Dimon 1) never uses the term "flash crash" and 2) spends the previous five paragraphs explaining exactly how and why liquidity dynamics in the fixed income markets have changed.

What he is saying is that the Oct 15th move looks really strange, but not if you actually understand how fixed income market makers are reacting to the Volker rule. Quoting Dimon:

For instance, the total inventory of Treasuries readily available to market-makers today is $1.7 trillion, down from $2.7 trillion at its peak in 2007. Meanwhile, the Treasury market is $12.5 trillion; it was $4.4 trillion in 2007.

What he's saying is that market makers have smaller inventories on absolute terms, and much smaller inventories on relative terms, so when multiple big market participants try to move in the same direction at the same time, there is much less inventory available to soak up demand.