This is true. People shouldn't just diversify for the sake of the word.
True. Diversification is a strategy and like all strategies these have to be moulded to each individual. There is not point buying anything you see just for the sake of being spread in shit markets. Wise choosing still applies.
Some people understand the word 'diversification' by exposing themselves not only to traditional markets in stocks and shares but also in the suit of strong projects in the crypto space. This is true diversification - or at least the way I see it

I think diversification goes deeper then just your holdings as well. Income should also be diversified. If you're investing into the crypto space for example, you likely don't want your entire investment based on buying and holding. You might also want to purchase some miners and mine Bitcoin, as well as some GPUs mining altcoins. You may want to advertise for services or sell your own products. You may want to stake some altcoins or provide liquidity pairs. You may even find yourself investing in DAOs, creating NFT collections or building projects in the metaverse. Going outside of crypto this same thing can apply. Most people buy stocks and bonds that pay dividends, but they could also be selling covered call options for additional income, or buy real estate and rent it out in addition to having a regular job. There are so many ways to diversify yourself. Do yourself a favor and do them all.
Good point, many times people think of diversification only in terms of their portfolio, but it is also important to diversify our income streams, many people depend exclusively on their jobs to get some money, but regardless of the amount they earn if at some point they are fired they will find themselves facing huge economic issues really quickly, however if instead we have two or more income streams if one of them dries up we have another ready to help us out soften the blow, until we can find yet another income stream to supplement the one we lost.