No, that really is not true. I am quite experienced in stocks and cryptos. I do some ta... and know how to "predict" the chart a bit but it is not always easy. But yeah, you never know how strong exactly a chart will go bullish or bearish (only circa) - cause of traps etc.
Nevertheless, everyone can make profit in a bullish market; only a bear market shows who is a good trader and who had just luck. Still there are people right now who just cannot trade.
However, you are right with hodling. Never sell all of your coins cause it could go up even when you do not expect it.