Suppose you learned, suddenly and definitively, that nothing is moral and nothing is right; that everything is permissible and nothing is forbidden.

There are different ways of understanding that. To clarify, let's transplant the thought experiment. Suppose you learned that there are no elephants. This could mean various things. Two things it might mean:

1) That there are no big mammals with trunks. If you see what you once thought was an elephant stampeding in your direction, if you stay still nothing will happen to you because it is not really there. If you offer a seeming elephant peanuts, the peanuts will pass through the trunk which is not there and will fall to the ground.

2) That big mammals with trunks are not elephants. If you see what you once thought was an elephant stampeding in your direction, if you stay still you will be trampled. If you offer a seeming elephant peanuts, the animal will accept and enjoy the peanuts.

Among those who would be persuaded that there is no morality, those who interpret the 'no morality' claim as analogous to (1) will change their behavior. Those who interpret the 'no morality' claim as analogous to (2) will not change their behavior.

(1) is a substantial claim about the world. (2) is a claim about language, about what how things should be labeled.

Those who claim that they would change nothing in their activity are treating the no-morality hypothetical as if it were merely a claim about how things should be labeled. Those who claim that they would change their behavior are treating the no-morality hypothetical as if it were a substantial claim about the world.