That isn’t a useful definition of racism. It’s sounds alright, although it’s ultimately idealistic, it doesn’t hold up when applying to material circumstances.
As for why people think having different rules for different groups is good, I think one of the simplest ways to sum it up is: Equality of treatment will not give equality of outcome until there is already equality of conditions. Treating all people the same isn’t fair in the real world.
As a thought-experiment to demonstrate: If we have two people, one has $200 savings after rent and the other has $10,000,000, you can’t make them more equal or make the money more distributed by treating them the same: if society wants to reduce poverty (which is obviously a good thing for society, to have less people in poverty), it makes some sense to supply the poorer of the two with money, but it makes no sense to supply the richer: they already have more money than 90% of people! There isn’t a moral or ethical benefit in giving them more money, they don’t need the money as much as others do, it’s not how to achieve fairness or equality.
The generalised point of that being, if a group is disadvantaged and the status quo is keeping them disadvantaged, solving that will require special treatment. Treating Indigenous people the same way as always just keeps the systemic racist status quo, and to solve that, the Government will inevitably have to treat Indigenous people differently. That’s a consequence of trying to create a more equal outcome in an unequal environment.
The same goes for other types of disadvantage, of course. I am obviously not trying to imply that all people who aren’t indigenous have all the advantage they need! Ultimately, everyone who is not a mega-multi-millionaire is disadvantaged, but we can’t fix that all in one change. We have to start somewhere.