Moreover, deploying ABM satellites using conventional warheads would not constitute a breach of the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies.
I didn't know that. I'd only ever heard of satellites being redirected in ramming attempts (i.e. repurposed as weapons).

There may be weapons systems in place then.

If we do have such a system deployed, it is unlikely that all of our defensive assets would be used absent a mass attack. I don't think anybody in our military is silly enough to deploy a single satellite with a pair of missiles and call it a defense. The US military does the tech thing pretty well.
But missiles are constrained by weight, size, and speed here. I doubt there are (or could be using 80s/90s tech) sufficient or sufficiently advanced weapons to counter a total exchange, given the limitations toward emplacement. So there's reason to believe a reserve would be largely depleted to defeat the NK arsenal. We might also expect anti-satellite missiles to be the primary weapon in orbit...