You showed the difficulties in diversifying between different coins, but you didn't explain why it is "bad". Done correctly, diversification is never bad.

If you want to diversify, you must choose non-correlated assets. Non-correlated coins do exist. Just look at the article's 2017 chart. BCH was not correlated with BTC, so diversifying into BCH was a good choice. This year, however, you will need to find other coins.

The article seems to say that you want a negative correlation in your investments. That is wrong. You want a correlation that is close to 0. A negative correlation may reduce risk, but it also reduces return. For example, the correlation between buying and short selling a single asset is -1. If you did both at the same time, your risk would be 0, but then your return would also be 0 (ignoring fees).