We should think more about the risk of having all your investments in one single portfolio, such as bitcoin. Bitcoin happens to be the most reliable cryptocurrency for investment but at the same time, Bitcoin is a highly volatile asset. So I will still advise you to diversify your portfolio into multiple sources.
I think he's just talking about bitcoin and other altcoins in the crypto space. It isn't wrong to put all of your money into bitcoin without using altcoins, which is something that a lot of newbies do not do.
Diversifying a portfolio from multiple sources is essential, cryptocurrencies are not a 100% safe environment and will always generate profits. So we will also have to invest in other areas besides cryptocurrencies.