I think we need to distinguish two different ideas here.
Yes, absolutely, "diversification" is a scam. This is the idea that you should always hold a variety of different coins, mid- to long-term, to mitigate the risk of one against the not-100%-correlated risks of others. Bad idea to hold shitcoins in your portfolio for 3, 4 years, by and large.

I think the post was getting at something else entirely. First off, putting 20% in shitcoins doesn't mean you think their risk mitigates that of bitcoin. (There is nothing with less long term risk that bitcoin here). But there are loads of normies out there who will "diversify" into shitcoins, shoot up some by 100, 500, 2000 % before the rugpull. Argument one would be: let those normies pay for my bitcoin.
Of course, this makes you a gambler.

Argument two, and I think this is what he's getting at: the normies need not be normies forever. They'll come to the space, think like stock market normies, diversify, hold 10 coins for a while and realize that bitcoin might crash, but comes back, while the rugpull shitcoins rarely come back. So they'll be burnt to understand where the value is here. That might happen, but it needs a few years. In only half a year, they might experience a 50% crash in everything and say, all of this is a scam,, I'll leave it. The media certainly tries to encourage this reading.

Difficult, though: you can't expect normies to understand it right away. As they say, you have to pick people up where they are, and everyone's living in fiatland. That bus is slow.