I think the reason Zealandia is called a "submerged continent" is because it is made of continental crust rather than oceanic crust.
But IMO the best geologic definition of continents is by tectonic plates, which mostly matches up with the cultural definitions of the continents.
For the major continents, we have these plates:
North American
South American
Eurasian
African
Australian
Antarctic
There are several smaller plates too, like the Caribbean, Indian, and Arabian plates. IMO, we should consider these independent continents.
There is also a dedicated Pacific plate. The ring of fire is the border of this plate.
New Zealand / Zealandia is on the ring of fire. Half on the Australian plate, half on the Pacific plate. You can actually see the border of the two plates when you look at the topographical map of Zealandia.
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.
Nope, they're all around you and me and everyone else. They just evolved into things that (usually) have wings. In terms of phylogeny, they're dinosaurs.
Well, maybe in theory. NZ has many unique birds not found anywhere else, but they are generally threatened or endangered. If you want to see them in the wild, generally you have to go to a very specific location.
If I look outside, almost certainly all I'll see are European dinosaurs.
So maybe I should correct my statement to say they almost all died out.
That Civilization 6 uses geological continents when the 'continent' key word is used where every other game in the series uses geographical continents for that key word still bugs me.
Seven is wrong no matter your definition of continent, unless you count New Zealand or "because racism."
Depending on your definition there are between four and nine continents, but the definition that includes Europe to make a total of seven necessitates India being its own continent without racism being the primary reason why Europe is a continent and India is not.
(Also if it transcended culture it wouldn't depend on culture for the answer)
(Also some people do say there are seven, because India is a continent with a land barrier and a tectonic plate and Europe is not)
And that's valid, if your definition is just continuous land masses. However, some definitions incorporate ideas of notable narrowings such as the African and Central American connections and continental plates, or even projected drift.
For example, some day the Americas and Africa/Eurasia will separate from continental drift. Some even argue the Americas already are separated thanks to the Panana Canal, humans just hurried the process along.