It means if it's possible a Creator can create morality without it already existing, it can decide what it is.
If can decide what it is, it can decide it's good and right and moral to torture a being for no crime forever and ever.
However this doesn't assert that it's actually possible such a Creator to exist.
Rather, the hypothetical is used to show it's impossible when it contradicts the later premise.
It's not in any possible world the case that it is good to torture a being forever and ever for no crime.
In short, it shows that it's not possible for a hypothetical Creator to do that in any possible world (ie. create morality without it already existing).
If can decide what it is, it can decide it's good and right and moral to torture a being for no crime forever and ever.
However this doesn't assert that it's actually possible such a Creator to exist.
Rather, the hypothetical is used to show it's impossible when it contradicts the later premise.
It's not in any possible world the case that it is good to torture a being forever and ever for no crime.
In short, it shows that it's not possible for a hypothetical Creator to do that in any possible world (ie. create morality without it already existing).