This is such a weird, pedantic definition of 'empty'. When people say a place is settled, they don't mean there is no room for another human being.
Europe was also mostly empty in this sense in the 1930s. Why was there this big war, the Germans could have just walked into those big empty spaces in the Ukraine and built some farms.
Wait; your own logic was "the continent wasn't empty, therefore, the only way to settle it was genocide". So, in your own logic, there wasn't any possible place left to the settlers.
Europe was also mostly empty in this sense in the 1930s. Why was there this big war, the Germans could have just walked into those big empty spaces in the Ukraine and built some farms.