I think the idea is that someone buys the "number" with 5m to turn it from dirty to clean. The person with 5m can just point to the purchase as the source of funds, and they don't have to account for where the money came before that.

So you have 5m in dirty money under a mattress. You buy crypto with it. Put up a 5m NFT, buy it with the "dirty" crypto, then cash out the crypto into a bank, and no one can prove you were the original owner of the crypto, so it's all clean.