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.
Newbies or Old timer , yeah there are some decided to buy purely in bitcoin but there are also many that desires buying multiple coins so maybe this is other part of the discussion.
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.
though there are some who only invest in Bitcoin but as majority of advices you get now are saying it is always best to diversify our folio , from buying one coin and another for holding long term.