It's faux wisdom.
What's a basket? You put yourself through medical school for 8 years, specialise in dermatology, have you put all your eggs in the 'Skin' basket? Or the 'medicine' basket? If you buy shares in a company which makes chemo drugs, are you diversifying into oncology, or are you still putting all of your eggs in the 'medicine' basket?
Change focus entirely and buy shares in a publicly traded construction company in your home city, are you still putting all of your eggs in the 'Baltimore' basket?
If I hodl a range of crypto across PoW, PoS and DPoS, is that diversifying into different chains, or focusing too narrowly on 'crypto'?
This bromide never defines a 'basket', so it means whatever you want it to mean.