The OP states it was part of Gondwana, maybe that’s what makes it different.

If you click through to the microcontinent link that seems to support the idea of microcontinents being pieces broken off a bigger one. But with everything coming from Gondwana then that means all the existing ones are fragments, and the only reason other fragments aren’t considered continents is size (e.g. Madigascar).

Zealandia seems to be the Pluto of continents. Too small to be a continent but much larger than the largest microcontinent.