There are some experience traders that knows their ways around trading and they also know the potential of Bitcoin investment, so they invest and hold for long term even though they are into trading.
How can a real investor combine investing and trading at same time, don't you think that there may come a time when he might decide to take part of his hodling to trade which can lead to losing all their investment if the trading doesn't work as they expected. Anyone can go for which they think works for them but combining investing and trading is not a good idea.
And because of this kind of Mentality, they can never achieve anything good. They get overly worried about a very small dump, and decide to stop holding, They seem to know nothing about Bitcoin, that this kind of volatility of Bitcoin is a normal thing. When the price drops for a while, they panic and sell their holdings at a loss.
No, it's wrong to say that traders cannot achieve anything good, perhaps some of them also tell about how they have made a lot of profits within their time of trading of which it can be true but the reason why we should stay away from trading is due to the high risk situation of losing all your money within a blink of an eye, so you can make profits now and in your next entry you run into losses and the sequence continues like that which may not be a healthy practice because there is no way emotions will not be attached to someone who is into trading.