I agree with this opinion, investing should be diversified in several coins. Bitcoin is a priority and we can choose several potential altcoins, I only recommend top coins, because I don't invest in new coins.
Diversification is a good way in investment. In this way, we can optimize the chance of getting bigger profits. If we only hold Bitcoin, we may miss the chance to get higher profits in altcoins. We know sometimes altcoins can increase more significantly than Bitcoin. Also, this way minimize the chance of severe losses. It is because we may hold some trusted coins, we don't only invest in shitcoins or low fundamental coins or hype coins. If we only focus on one of the type of the coins, it is possible to lose all the money. Moreover, if we choose hype coins, shitcoins, or new meme coins.
However, it is not a must to do diversification.
If you think it is enough to invest in a single top coin only, no problem to invest in it.
For example, we want to focus on BTC or ETH investment only.