It explicitly doesn't say.
But -- as per @ZhonLord -- the coins are meant to be in circulation, not in a vault. I suspect that not even Hades' security is sufficient to overcome this.
And -- as per @Arjan -- there was plenty of time for Nic/Anduriel to recover Lasciel from the rocks, and then Ursiel from the ice.
Last but not least... I expect Jim has some stuff he's planning to write in a Harry/Bonny/Lasciel arc ... The whole "child shared with a hostile Ex" thing seems like too much of a chance to torment Harry; and tormenting Harry is most of how Jim stays fed & housed these days.