They're confusing migration (violent or otherwise) and imperial urban rule with settler displacement and the annihilationism that goes with it

Also the defining character of settler colonialism has been its use of industrialization and market building to shore up a racialized ideology and vice versa

That was rarely or never the case with the old empires which were agrarian tribute systems that usually relied on their subject peoples being semi autonomous to relive pressure on imperial bureaucracy, that's not to say there wasn't settlers, it simply means the ruling mode of production didn't always cater or center around them

For instance, if the Roman Empire had been settler colonialist in the capitalist sense of today, Greek wouldn't have remained the lingua franca of the eastern half of the empire, and every Roman emperor would've remained an Italian or "ethnic" Roman