They want to express hypocrisy. When someone omits that other groups exhibit the same behavior as the group they're complaining about, it's dishonest to attempt to file a grievance without disclosing full information. Consider it a lack of "informed consent," to borrow a medical term.
But it can only be hypocrisy in the case of (2). So whenever someone invokes whataboutism, they mean "Native Americans and settlers both are evil"?
So they agree with my point about the genocide of Native Americans, the stealing of land, oil, gas, gold, but want me to add "Native americans are bad too because they did the same" which I should have mentioned.
Therefore they would be fine with "Modern Americans are rich because they stole the land, gas, oil, gold from the Native Americans, who stole it from someone else before".
I always thought they considered their opinion a counter argument, I never saw it as agreement with my points.
1. Native Americans and settlers both did it, so it is good?
2. Native Americans and settlers both did it, so both are evil?
Is it 1 or 2 that you want to express?