For a wet cure (pickle) that is a small amount of TQ for 1.5 quarts of water, TQ directions are to add 1 cup per quart for a brine cure. It looks like the recipe is using an amount to provide some protection without altering the taste of the fish. I'm more concerned about the amount of sugar vs the small amount of salt that TQ will provide
. Out of the 2/3 cup or TQ, you have about 1/2 cup (≈5oz) of salt. So you have 2 times the sugar as salt.
I generally cook salmon until it reaches 140°F, but others like their fish drier.