I don't think it has any problem, if we hold ETH and bitcoin for a long time, these are basic coins of crypto and if we want to big profit from them, then we must need to hold them for a time.
As you said sell the ETH and bitcoin when market down, i also disagree with you, when market down then we sell them, we are doing double mistake. When market down just hold them, with a passage of time, definitely ETH and bitcoin give us triple or more than it profit. Now you could see the bitcoin price has started to go up.
He is just trying to corrupt the mindset of those that will hold their coin with his own impatient. Holding depend on the level of patience and strategy of each of the investors, not everyone can trade or do shorts one like he said, the most important thing is that everyone benefits from their investment.
So,holding to me is still the best option for investment in cryptocurrency, especially when the investment is being done during bear market, it will really end up given a very great profit in future, it is best to make short trades when it is the only source of income for one but if it is not, then long term option will be best.