You are misreading this. The Dormant Commerce Clause is relevant when talking about applying laws unequally for the purpose of protecting in-state commercial entities. For instance, if companies based out of state were subject to salary disclosure laws, but Washington based companies were not.

In this case, Dormant Commerce does not apply since the salary disclosure laws apply to any company operating in the state, regardless of where they're headquartered or incorporated. It applies equally to all.

That's specific to state protectionism though. The law applies equally to jobs posted inside and outside the state so the Dormant Commerce Clause is not relevant.