In the same fashion we can declare pretty much all words messy, simply because their meanings are context-dependent. It's true even for majority of hard science terms. However, it doesn't make that postmodernist way of cancelling words any less absurd as it makes all discussions impossible.

As a good example you yourself use "native" which is as messy, and complex as "ethnicity". You arbitrarily chosen Columbus arrival as definition point, which can make you feel comfortable in the moment, but it's neither universally accepted, nor universally applicable. Using the same logic it must be banished from serious discussion.