Reparations are meaningful only in a context of liberalism and capitalism (or some similar system of private property). But liberalism and capitalism (or any other system of progressive aggrandizement of property) imply imperialism. Therefore, reparations and anti-imperialism are, if not contradictory, at least going in very different directions.
As one example of how this difference could play out in events, consider that the American ruling class might be quite willing to pay reparations if, following Machiavelli, they could steal the money or goods from some other people or country. (Machiavelli said that princes should never give anything away unless it belonged to someone else.) At home, the wealth could be arranged to flow from one segment of the bourgeoisie to another, possibly new segment, without much touching the lives of those who had been suffering the most from the crimes done to their ancestors.