"The UNSC doesn't have the power to set universal international law" - no one does, and no one is claiming otherwise. Why are you arguing against a strawman?
So, for the UNSC to have the authority to make it illegal under IL for NK to develop such weapons would be tantamount to it having the authority to decide universal international law.
So unless you can find a chapter referred to by resolution 1718 that gives the power to ban the development of some types of weapon, the UNSC doesn't have the authority to make a country's weapon program illegal. All it can do is implement certain sanction or make military action legal.
In any case, if the UNSC did have this power, why would the NPT need to exist? It could just decide that developing nuclear weapons is illegal for here on out, no need to get countries to sign a treaty, right?
I think what we have here is a disagreement in terms. There is no such thing as "universal international law". Period. Because unless there's a mutual agreement by all countries to accept some sort of governing body, over their own local law, and to supply enforcement, it can't exist.
That said, there -is- an international body, that has had most countries join, with a charter that stipulates certain things that body can decide, and how those member countries promise to respond.
That body's charter says "Members agree to accept and carry out the decisions of the Security Council". DPRK is a member. Ergo, it agreed to accept and carry out the decisions of the Security Council.
A decision of the security council was that the DPRK needs to "suspend all ballistic missile activities" and to "abandon all WMD activities." Ergo, that body has decided the DPRK is prohibited from doing those things (the original discussion, remember?). The DPRK has not done so.
Back to the point of this whole thread, the consequence of their not doing so is that member nations are forbidden from doing certain things for/with the DPRK ("sanctions"). Which is what this document is about. To say "how can it be 'prohibited' for the DPRK to engage in ballistic and WMD activities", -that's how-. That's the point of all of this. The UN prohibited it. And stipulated sanctions, which the US is adhering to.
"Well, that's not -illegal- then!" - what does that even mean in that case? "The DPRK didn't agree to -this- particular rule" - so what? I didn't agree to the speed limit; if I decide not to obey it, the US government can still enforce a punishment on me. I'm not really sure what you're arguing, except that the UN doesn't constitute "universal international law", but I don't even know what that means; certainly, whatever it does mean, given your objection, it doesn't exist, so I'm not sure why you'd think "prohibited" must mean the same thing as it; what are you trying to argue? The DPRK has not upheld the things it agreed to when joining the UN, the UN responded, and that response (and what it means for US based corporations) is obviously what is being referred to in the PDF.
>> In any case, if the UNSC did have this power, why would the NPT need to exist
Because ultimately the UNSC will not agree to non-proliferation. The NPT has no enforcement provision for those who do not sign it, but becomes a way to establish international norms for those who do. In short, the NPT is a way for a country to say "I believe non-proliferation should be standard, and will agree to not do it"; it is opt in, only applies to those who opt in, and any country can opt into it (or out of it). The UNSC is force; an agreement there can be decided on even against the wishes of another member state (as happened with the DPRK), just by dint of that other country being a member state, and carries with it penalties such as sanctions.