But we're speaking hypothetically as if they were actually buying bitcoins, but not withdrawing them, because it's cheaper in the long run to pay with other currencies.

But if you never withdraw them, they have no value at all. Therefore some are withdrawn, and at least some of that $1 came from bitcoins. If you even withdraw one penny worth, you have added one penny to your net bank account, which has influence on your account forever.

The simpler hypothesis is that they intended to show the uselessness of a deflationary currency by infinitely hoarding. But that's completely irrational, so an alternate, simple hypothesis is that they are using bitcoin, so any money they have is influenced by bitcoins they exchange.