The concepts of assets diversification is quite self explanatory though a lot of people understand it differently but just as it implies it entails roll out your investment around difference and unrelated asset classes, revolving around Bitcoin is never diversification, revolving around Bitcoin and other coins is not diversification either but when you are revolving around Bitcoin and others such as real estate, stocks, or selling of commodities it can be considered as diversification because they entirely not related.
I disagree with you, diversifying into crypto after investing in Bitcoin is still diversification but the thing is, it is not advisable to diversify into crypto again when one have already invest in Bitcoin and In the crypto space the only reliable coin is Bitcoin so Investing in any other coin is a more like a waste of time because they are not reliable and can easily mess someone up. Real estate and Land is a very good investment because they are asset that bring value, not just value but great value after a while and there is no too much factor or barrier.
Stormisover is correct. When you are investing into bitcoin and start putting part of that money on shitcoins is not diversification. Diversification is done for by spreading your funds into various assets of different classes and not of the of the same class. The reason is because it's for security and reduces the risk of only investing in one asset.
If you invest in bitcoin and shitcoins, when bitcoin dips, all your shitcoins goes down with bitcoin increasing your risk. But if you invest in bitcoin and stock is bitcoin price dips, stock might not dip or rather the price goes up. Diversification is to balance and manage risk.