Assimilation empire wants you as a subject. You get pulled in, taxed and ruled and absorbed, but still standing, still a people, still there when the empire eventually falls.
Settler colonialism is the other thing. It doesn't just take the land, it clears the people off it. Massacres, forced marches, children torn out of their families, whole populations starved and hunted and penned onto scraps of what they used to hold. And it grinds down whatever survives, the language, the culture, the ability to pass any of it down, until the remnant is too broken and scattered to organize or reclaim what was taken.
One wants you to share its identity. The other wants you gone, and kills and breaks its way there so you can't fight the going. And the settler countries built that way are still standing on the results, land never returned, treaties still unhonored, the whole thing settled comfortably enough that they can lecture everyone else about human rights.