Russia argues that it's defending the self determination of ethnic Russians in the Ukraine.
Maintaining the current borders does not advocate for self determination. War might not either, but the Ukraine isn't going to give up that land peacefully
First, it's not exactly clear that those ethnic Russian regions wish to be ruled by the Russian state. Second, Russia is in the midst of a full-blown invasion of all of Ukraine, and Putin has claimed that all Ukrainians are in fact Russians:
Maintaining the current borders does not advocate for self determination. War might not either, but the Ukraine isn't going to give up that land peacefully