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.
i disagree.
altcoins in the long run, they all act the same meaning they go down whenever bitcoin is active. and BCH has not been an exception to this rule either. the difference is that BCH has been new, controversial and advertised to death in the short time it existed so it might have acted a little differently from other altcoins.
In the particular case of BCH I agree with you. I think the reason is simply that BCH doesn't differ from other altcoins in that its trade pairs with Bitcoin are the dominant trade pairs, and so it "costs more" to move the price against Bitcoin (buyers and sellers would have to move their orders) as to "keep it in sync".
There are however cases where some currencies seem to have achieved a sort of "decoupling" from Bitcoin's price. Decred and Faircoin come into my mind, which seem both to be significantly less volatile than BTC. I'm only speculating about the reasons, but Decred has an internal "ticket" market which may be stabilizing the currency as it "freezes" a relative large amount of coins for some time (and so helps to prevent massive panic sells to a certain degree), and Faircoin has a big ecosystem of merchants and some additional, a bit centralized mechanisms that control its exchange rate (for example, a voting system to decide a peg to fiat currencies).
If coins with this kind of mechanisms like Decred and Faircoin, which reduce its dependency to the Bitcoin price, get more popular, then large-scale diversifying might begin to make sense even in bear markets and not only in some bull market phases like I wrote earlier.