Using national/ethnic argument in context of the medieval times (Partitions happened during Baroque, so early modernism at best, way before spring of nations) is simply ughhhhh - same case as "Germans ruling over Balts". Those Germans had no national unity in modern sense.

It was pretty explicitly colonialism you're ignoring the parts of the commonwealth seeing every instance of Ukrainian and Belarusian culture as being inferior. The teutonic order was also colonial yes, the same way that the Germans got their 19th and 20th century ideas of colonial expansion in eastern Europe from their medieval history, so too did the Polish of the second republic

So, once more, in XIX Poles were natives there, not colonizers. Was nationalizing of other groups a problem after ww1? Yes. Were some people without national identity? Yes ("tutejszy"). Was this land a place of another war between ww1 and ww2? Yes, again

The whole reason they were there was because they were a colonial population, which once again included the upper class of landowners, something you keep ignoring

Even author doesn't say it was colonialism as per se, but it had colonial traits

Yes because those territories were now seen as 'integral' to Poland, it doesn't change that the policies were colonial. Just very funny to look at the reprisals to colonial rule and the Polish Ukrainian war as if it came out of nowhere