I don't support ethnic cleansing and neither would Zelenskiy, however there are many Russian citizens living in Crimea now, not former Ukrainian ones. Ukraine is a multicultural liberal democracy, but feelings are running high there now and unless some are prepared to swear loyalty to obtain citizenship, I see no other way. I expect a large influx of Russians after the war, economic refugees, Ukraine contains Russian and Ukrainian speakers and the difference between the languages is not great.
The point is not ethnicity or even language, it is the political views and loyalty of the citizens to the state and every state must guard against potential 5th columns, even America. If Russians living in Crimea want to be citizens, then they must eschew Russian citizenship and imperialism. This is really a legal question for lawyers and judges to decide and Ukraine has the rule of law, unlike Russia.