In a 2015 Quanta article talking about a breakthrough in the graph isomorphism problem, it was mentioned that graph isomorphism has a unique property where we have an interactive proof protocol for it. That is a prover can convince a verifier that two graphs are not isomorphic in polynomial time. That is, for the graph non-isomorphism problem a prover can convince an efficient verifier that two graphs are not isomorphic. We note that graph non-isomorphism is not known to be in either $NP$ or $BPP$.
But (as indicated by the Quanta article), is it the case that we have no known interactive proof protocol for NP-complete problems? That is, do we not have interactive proof protocols for problems like say the complement of Subset-Sum, where a prover needs to convince a polynomial-time verifier that there is no subset of the problem whose instance equals a target value?
And moreover, what would be the implication if there was, or was not such an algorithm for an NP-complete problem? If my understanding is correct, if there was such an algorithm, then it would prove that $coNP \subset IP$, but this is already known since $IP =PSPACE$, so I suppose I am confused on why this problem would be a big deal or not.