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

This is more medieval than ancient, the Crusader states in modern-day Palestine and Lebanon might be considered settler states, and Germans settler-colonized East Prussia (modern-day Kaliningrad) and completely wiped out the natives.

Even if settler-colonialism was common throughout the ancient world, that doesn't excuse it happening today. Remind them that rape and slavery were common throughout the ancient world as well.

The point is that collectively speaking, the vast majority of humanity rightfully agrees that rape, genocide, slavery, colonialism, and unprovoked murder should belong in the dustbins of the past, to not be repeated going forward.

Israel, the U.S., Britain and India (the Kashmir region is arguably Indian colonialism) are the only or primary countries still engaged in settler-colonialism, and actively holding humanity back, and these countries must be completely destroyed or rebuilt into socialist states.

Before nationalism and monotheism, as far as warlords were concerned a peasant was a peasant; what language they spoke did no impact their ability to plow. As such, Settler Colonialism was rare but not unheard of. Best examples are the crusades, Germany's colonization of Prussia (see Baltic crusades), Japan vs the Ainu, and the Bible (Jewish conquest of philistine).

The far more common occurrence was that invaders would replace the ruling/warrior class with their own people and the commoners would be left to work. Sometimes the two classes would form a common culture (Saxon invasion of England, Norman invasion of England, Roman Europe, French invasion of Gaul), other times they would not (Ottoman Turks outside Anatolia, the Selucid Empire).

What even was the point they were trying to make? It happened before and that makes it okay now?

Perhaps everyone did but the Palestinians surely didn't, so why are the Palestinians being punished for the crimes of other nations?

Ben-Gurion wrote:

The fellahin are not descendants of the Arab conquerors, who captured Eretz Israel and Syria in the seventh century CE. The Arab victors did not destroy the agricultural population they found in the country. They expelled only the alien Byzantine rulers, and did not touch the local population. Nor did the Arabs go in for settlement. Even in their former habitations, the Arabians did not engage in farming ... They did not seek new lands on which to settle their peasantry, which hardly existed. Their whole interest in the new countries was political, religious and material: to rule, to propagate Islam and to collect taxes.

Source: David Ben-Gurion and Yitzhak Ben-Zvi, Eretz Israel in the Past and in the Present

Colonialism is how we got capitalism developed as a mode of production, consuming others in the historic centers of human civilization. Spread from Europe into West Asia, India, and China. A millenium ago, so recently rly

This is a super lazy answer, but the Wikipedia page on this lists nearly only European colonisation on non European peoples. The few counter examples seem to be Morocco, Indonesia, and Japan.