That statement only for people who have patience in the long run and wise when market is very tricky. When the time during bull run, some still don't want to sell at a very huge pump expecting to raise more price. Now ending up cannot sell now due to dump price. Holding is also good but if your greed is too much then t wouldn't help at all. Learn to be contented.
that is all true ONLY IF the thing you are buying or holding already has a good potential of rising. but if it doesn't and if you are fooling yourself into believing it has potential of getting pumped then you will end up wasting a lot of time and money on it.
for instance there are currently a lot of newbies who are bag holding a lot of ICO tokens that are continuing to die!