In my understanding, it seems that he means to buy when the market is low because of many fearful sells and sell when they are greedy meaning their demands are high so hence the high prices. It applies to cryptocurrency as it follows the basic rule of economics also.
Typically, when others are fearful because of often dumps unlike when we go to 5,800 everyone is fearful in buying that is a big break for the big investors and whales to buy and when the market pumps like the recent high everyone is greedy to pump it more where the big investors sell on that point.