This is not entirely true either. If you genuinely understand an asset deeply, concentrating on it can sometimes generate much higher return than spreading investment.
It depends on ho you view it. Diversification is surely very possible to increase the chance of getting profits if you choose the right assets.
Please read carefully what I said above, I said "choose the most reputable assets only such as Gold, Bitcoin, or properties".
I believe that going all in on Bitcoin can sometimes yield superior return compared to diversifying into gold or real estate. However, that is too risky and it is more like gambling.
If you are real investor, you won't invest with a gambling approach.

Even if Bitcoin can increase higher but it takes few years for rising significant. If you only invest in Bitcoin, you need several times to be able to take profits. But if you also invest in Gold or property, you may take profits faster. And they are also safe assets for a long run. So, there is nothing wrong to diversify assets on them, right?
Diversification aims to minimize risk, not to help us earn greater profit.
Sure, it minimizes the risk. But it also can rise the chance of bigger profits, too.
I even think we should prioritize income and emergency fund before thinking about investing. Investing is the next step, not the first.
If you read carefully, you will be aware that it is the point of my post above. People can invest if they have stable income, which means they have no problem anymore with their monthly income and emergency funds.