Agree with that.
Thing is though, as you say he is eligible for NZ so long as he has the certificate so all the reporting that he isn't eligible for NZ on a correct interpretation of the law is wrong.
Can NZ make an argument from that? Yes I think so. Will it be completely convincing, who knows - will depend on the facts. But if FIFA asks you to state your intention to play for your adopted country, and you've already played for them...or what if we'd been informed by OFC that this was an issue and had time to make the request from FIFA? There are lots of scenarios that could play out here