Ya but with BCH, being a hard fork where the asset was created from everything to a certain block of the BTC chain, it effectively created new coin balances out of nothing. Now imagine when people start trying to cash those out to fiat.
In this situation, there was no fiat laid out to get it but it is indeed worth fiat in the case of a withdrawal.
If people want to cash out their BCH, they need to sell it on the exchange for USD. The USD needs to already exist in the exchange for someone to buy it. The fiat was laid out by the buyer.
So there's no fundamental liquidity issue, if everyone wanted to cash out their BCH to fiat it should not be an issue.
Because they just bought it from you at that guaranteed price. They could go and sell it on the market for more/less, but the price you sold it to CB for remains the same.
In this situation, there was no fiat laid out to get it but it is indeed worth fiat in the case of a withdrawal.