I don't eat soy, but I've long wondered how and when I'd add it to these nuts because I know tamari nuts are a (expensive) thing, and since tamari is basically just salty soy water it's a natural substitution in this recipe.

First, the water is hot to help aid the salt in dissolving. It doesn't sit on the nuts long enough for it to penetrate past the skin. Not like some roasted nut recipes that have you soak the nuts in salt water for several hours first. Those usually have a long cooking time at a low temperature in order to get all the liquid out. This recipe uses the salt water as a way to get the salt to stick to the outside of the nuts, though a lot of it still falls off during the cooking process.

Since tamari is already salty, I think you could skip the water/salt step entirely and just toss the raw nuts in the tamari. I don't know what happens if you roast tamari sauce, though--is it going to make the nuts burn faster? But if you add the tamari to the nuts after they're roasted, will it soak in? And if it does, will it make them soggy?

An experiment is definitely called for.

I'd start off with replacing the salt water with around a tablespoon of tamari, roasting the nuts, carefully tasting one, and then oiling them, giving them an extra hit of salt if necessary, and sprinkling on the ginger.

If it works, awesome! If it doesn't: Plan B. :D