That seems like a purely semantic argument. If a bunch of people from a particular political movement show up somewhere and seize a bunch of land and declare it's part of their nation, whether that nation is completely new or existing doesn't change the morality of that anywhere. E.g. the formation of South Ossetia on Georgian territory is widely recognised as either annexation or something morally equivalent to it.
> Generally international law recognizes the right for ethnic groups to self-determination (Palestinians often rightly complain that they effectively have not had this in the modern period, but the right applies to both sides). It is not considered annexation for a group from one state to split off and form a new state via self-determination.
That's arguably true for people who declare independence in the territory they're already living in. Not for people who settle on someone else's territory and then claim it as theirs.