Natural law theory accepts that law can be considered and spoken of both as a sheer social fact of power and practice, and as a set of reasons for action that can be and often are sound as reasons and therefore normative for reasonable people addressed by them. This dual character of positive law is presupposed by the well-known slogan "Unjust laws are not laws." Properly understood, that slogan indicates why—unless based upon some skeptical denial that there are any sound reasons for action (a denial which can be set aside because defending it is self-refuting)—positivist opposition to natural law theories is pointless, that is redundant: what positivists characteristically see as realities to be affirmed are already affirmed by natural law theory, and what they characteristically see as illusions to be dispelled are no part of natural law theory.
I've said similar things before. Natural law theory does not require the claim that there is no positive law, nor the claim that positive law is natural law, but only the claim that legal positivism is not and cannot be an adequate account of law, even positive law. (Later in the article Finnis notes that thinking that 'An unjust law is not a law' is somehow a contradiction is in natural law terms like thinking that 'A quack medicine is not a medicine', or 'A disloyal friend is no friend' is somehow a contradiction; it's either perverse interpretation or a failure to understand how language is able to work.)
One of the strengths of the article is that Finnis explicitly talks about Aquinas's notion of determinatio. Aquinas notes that you can sometimes derive conclusions from principles without directly deducing them; this occurs when the conclusion is a more particular instance that, so to speak, falls under the principles. Thus, the conclusion depends upon and derives from the principles, but is not necessitated by them, because it also depends on contingent facts. For instance, rules governing the acquisition of private property are not fundamental moral principles; rather, they derive by way of determinatio from fundamental moral principles, contingent facts, and prudential judgments about the best way to conform to those principles given those facts.
Finnis also rightly notes that in natural law theory, natural law is always something that must be operative concurrently with any positive law reasoning. You can have natural law independent of positive law (e.g., in the absence of any legislation), but you can't have the reverse, because positive law is connected to practical reason by way of natural law.
While the article restricts itself almost entirely to issues raised by legal positivism (since the vast majority of contemporary discussion surrounds these issues), and therefore does not capture the full power of natural law theory as a way of approaching questions of law, it does do a very good job with those.