I see your point, but I don't consider what I said to be contradiction. First of all, I was speaking only in regards to stocks, which do not comprise 100% of my investments. I also own bonds, real estate, and crypto as well as short-term notes. The diversification was only in respect to the equity portion of my investments.
So I suppose there are two types of diversification, and I was speaking about the smaller set. There's diversification across investment types (what the asset is: stock, bond, real estate, commodity, crypto, etc.) and there's diversification across industries which is most commonly (but not exclusively) used in relation to equity investing. If you're invested 50% in stocks and 25% in bonds and 25% in real estate, and all your stocks are in one industry, I wouldn't consider that diversified. You could be well-diversified inside crytpo (10% across 10 different coins) but equating 100% of your investments and I wouldn't consider that diversified in the larger sense either, since crypto by and large is dominated by btc, and whatever btc does it for the most part drags every other coin with it. In equity investing, there are defensive stocks that perform better in recessions than growth stocks which are very dependent on good economic conditions. So there are several ways to get at diversification, but in each case it's going to depend on the facts of the case. I would say a general way to put it is you're diversified if a general event doesn't impact all your investments at the same time. (Exceptions for catastrophic market events, which will drop everything, because you can never be diversified to the point where you never experience losses.)
As to me, this kind of diversification doesn't make a lot of sense for an individual
Note that I specifically point this out since a big investment company (like Berkshire Hathaway) is a totally different matter (obviously, they have quite a few of different individuals). Diversification, as I understand it, serves for profit multiplication at a certain level of risk. If you are looking for capital preservation, you buy gold, and that's pretty much it. Since no one can learn and understand any asset perfectly, it makes sense to invest in no more than 3-4 assets (as I told before) while learning these assets as deep as possible, otherwise you will be wasting time and energy mindlessly shuffling your money between different assets with no real purpose