I didn't say ”imperial concept”, its an “age of imperialism” concept (though, in retrospect, “age of exploration” is when it started, it just really gained salience in the age of imperialism; though whether it was ~1300 or ~1600 years too late to apply to him isn’t a big difference.)
At least in the late Roman Republic, there was absolutely a concept of race that unified e.g. the various Gallic tribes, or differentiated the peoples of the Roman East. It's always been a sociopolitcal concept. But the Romans were aware of e.g. North Africans versus dark-skinned Africans.
Why wouldn't imperial social constructs apply to a literal emperor?