is certainly not unambiguously libertarian

It is. It strictly is, in the very sense of the word, a Libertarian policy:

• Adam Smith: "The first duty of the sovereign, that of protecting the society from the violence and invasion of other independent societies, can be performed only by means of a military force."
• Von Mises: "The peoples who have developed the system of market economy and cling to it are in every respect superior to all other peoples. The fact that they are eager to preserve peace is not a mark of their weakness and inability to wage war. They love peace because they know that armed conflicts are pernicious and disintegrate the social division of labor. But if war becomes unavoidable, they show their superior efficiency in military affairs too. They repel the barbarian aggressors whatever their numbers may be."
• Von Hayek: “I am convinced Reagan is right not to reduce arms expenditure. World peace depends upon America staying strong. We already have so many atomic weapons that a nuclear war would mean the end of civilization: so the discussion as to whether arms increases intensify the threat of war is nonsense. In fact it’s no longer a question of whether nuclear war can be avoided or not; the real problem is whether we have got ourselves into a situation in which the Soviets can intimidate us to such an extent that we knuckle under completely. We can’t afford that kind of weakness. Ergo, the West must stay at least as strong as the Soviet Union."
• Milton Friedman: "Unfortunately, we need to have an army, and we need to have a good army and an efficient army."
• Gadsden Flag: The basic principle of being able to defend yourself is the one outstanding symbol in the liberal flag, underscoring it's core importance.

I wouldn't say this is the author missing anything.

Right above I have just showed he is missing everything, literally to the point of being both laughable and lame. To start, he is missing what's the meaning of Libertarianism and what it stands for. He demonstrates in his article a profound ignorance on the most simple and basic principles of the libertarianism, and I have just demonstrated that with actual names and quotes on the above paragraph. It's there, it's all over the foundational bibliography, I have just quoted it. Now the author can disagree, but then he can't call his view to be "the" libertarian view.

many libertarians...

...are not "all libertarians" and much less are they "the Libertarianism" in itself, again, as demonstrated at the very beginning of this post.