That is another good question. You are right. All the zones with an ocean crust—and I am not speaking about the continental crust we live on—that are located on the seabed are international, for example the North Atlantic. The Eurasian basin to which I was referring is the real ocean crust. It is located beyond the continental shelf as it is perceived by any country. It would remain international.
In other regions where the crust is neither continental nor oceanic, but rather hybrid, from what I understand from the UNCLOS specifications, there is a limit to what a country may claim. If everyone were to claim as much territory as possible, even 350 nautical miles along the ridges and all the rest, there would still be large areas in the middle of the Arctic ocean that would not be claimed and that would remain international.