If it’s within 200 miles of land, there’s established protocol under the UN Convention on the Law of the Sea[0] to demarcate maritime boundaries based on land borders. If the border went through the island there would need to be some sort of treaty negotiation to establish sovereignty.
The short version is that overlapping maritime borders are resolved by drawing the various buffer zones out from the shoreline of all coastal land of each country, and where they conflict take the approximate midpoint of the overlapping area as the border.
Small islands can therefore have an outsized impact on extending the territorial waters of a country, which is why there are so many territorial disputes over seemingly-insignificant uninhabited islands.
There's not always an unambiguous resolution to competing claims, see for example:
but in reality if new land appeared in the middle of the English Channel on the French side, I can’t imagine the U.K. being happy with suddenly losing a fair bit of EEZ.
China can get away with it because might makes right.
[0] https://en.m.wikipedia.org/wiki/United_Nations_Convention_on...