"International importance" is a different argument than "national sovereignty", although you have correctly identified that the two share very similar playing fields. An argument that X resolution is "not an international issue" means 1) the issue does not inherently concern multiple states at once, in the way that oceanic oil spills and terrorism do, or 2) the issue is "not important enough" to merit international attention, in the way that the order of presentation of athletic delegations to international sports competitions is technically an international issue, but is in absolutely no need of international legislation. In contrast, the "national sovereignty" argument asserts that a given issue, international or not, must be left to the whims of member states, and that international legislation represents an unreasonable encroachment upon the right of member states to self-govern.
Both arguments can crop up in response to similar proposals, especially since proposals of questionable international importance tend to represent encroachment into policies that many member states would rather maintain full authority over. Even so, they are indeed separate arguments.