I was going to say that he might be allowed to get his NZ passport back now, but I suspect you're right. A lot of countries which ban dual citizenship (or just ban it with certain other countries) don't actually stop you re-gaining your first nationality once you've initially dropped it in order to get the sole nationality of your adopted nation. So to explain, if an ordinary New Zealander was to drop their NZ citizenship in order to get Japanese citizenship, there might not actually be anything stopping them regaining their New Zealand citizenship, as this is out of the hands of the Japanese authorities and they might not even know.
However, I'm not sure that would be the case here, as I think the slightly weird system of dropping your citizenship and subsequently regaining it is one based on your adopted country turning a blind eye, rather than expressly allowing this to happen. If this is the case you're right that they might not be so lenient if Fitzgerald was openly flouting the rules by representing New Zealand internationally in football.