everyone can profit from that saying, the trouble is not the saying, it is the fact that people can't do it. When bitcoin crashed to 4k in march, people sold because they thought it would go lower, hell some people actually sold so that they could buy a bunch of bitcoins lower. What happened? It went higher.
I am not 100% sure what you meant there, you are saying that people sold at 4k when the panicked but then you say they did that to buy at lower price but why would you sell BTC at 4k to buy it at 5k ? .. I mean this just doesn't make sense or I am sorry I failed to understand your logic.
A lot of speculators actually do this for making money and sometimes they get caught in the traps of trend reversal, when the price continues to fall for a long time then people think that the Price will fall to a lower level and they sell to buy coins at a cheaper price in order to cut down their losses and sometimes also to gain more profit from the bear market. Even a friend of mine sold bitcoins during the crash this March thought that he will buy once again when the Price will reach $3k like the start of previous year, but he was totally devastated to see the Price rebounding back to the same levels before the dump.