Overall the article is right - the market is still extremely inter-connected. That may have to do that almost all altcoins are traded mostly in Bitcoin.

But I've observed that the success of a "diversifying" strategy has to do with the phase the market is currently in. So a good observer can achieve advantages diversifying and "un-diversifying" at the right moment.

The stats in the article you linked, for example, show that altcoins often exaggerate mid-term movements. For example, the last bullish cycle in Bitcoin in late 2017 was crazy, but much crazier in most altcoins. In contrast, in 2018 most altcoins fell much deeper than Bitcoin.

This shows us that - in contrast to most diversifying strategies at the stock market - a "diversifying" strategy in cryptocurrencies is not a good idea if you want to protect yourself from price drops. Instead, just in market phases where it's clearly visible that a crazy bubble is happening, it's best to gradually "un-diversify" - that means: gradually swap your altcoins for Bitcoin.

There are very few examples of cryptocurrencies that did better in bear markets than Bitcoin (Decred, for example, is one; Ethereum at least didn't lose as much at others), but it's difficult to predict which they are.