How do you steal the gold Helmut?

Cloak on its own does not get you the gold Helmut. The helmet shells only respond while you are transformed into Wisp Dust and Cloak is active at the same time, so both have to be running when you reach them.

In practice that means setting up before you approach. Transform into Wisp Dust, switch Cloak on, then go to the shells and interact with them. If the shells give you nothing, check that both are still active, because the interaction depends on the pair together rather than on either one alone.

Taking the gold Helmut from the shells this way is what completes the quest.