Concepts taken as the basis of a world view not existing at all tends to throw people off as they cannot shake the later ideas of fixed hard boarders and citizenship by blood. To their worldview there should be a fundamental traceable line to back before these distinct entities existed. It is the us vs them line to many.

Their ideas of ethnicity are fundamentally lies which while old aren't as old as they claim to be. People tend to freak out when foundational beliefs are revealed to be lies regardless of how innocous - see the reaction to imaginary numbers, non-Euclidian geometry not only being proposed but proven to be real and the legendary Pythagorean freak out over pi proving that not all numbers are rational.