I think you're mostly right, but it gets complicated when you start to look at the distinction between equity (i.e. equal outcomes) and equality (i.e. equal opportunities).

The reason the pie gets bigger when you address racial disparities in an equality-minded way is that you only allow yourself to help people better grasp their opportunities. This means that, for the most part, you have to engage in interventions that allow people to better themselves. Bettered people means more pie--more brilliant scientists, more moving artists, more skillful doctors, more creative entrepreneurs.

But when you address racial disparities in an equity-minded way that does not align with an equality-minded way, you are typically not engaging in interventions that allow them to better themselves. For instance, if an elite high school with demanding academic barriers for entry does not have equal representation of different racial groups, and you decide to solve the problem by abolishing the academic barriers and admitting people in a lottery with numbers per race chosen according to demographics, you have achieved equity, but you have done nothing (at this point) to enable anyone. The pie is no bigger; it's just cut differently, at this point. (Maybe in the long run this will result in more pie overall, or less overall, but unlike with equality, it's not a clear win.)

I think most interventions thus far have been equality-focused, and therefore your characterization in practice is mostly correct: the pie gets bigger. Indeed, this is the premise of the entire liberal democratic system--and a premise that has proven solid for the most part--that when you enable all your people, everything gets better for everyone. The process goes: you notice inequity and ask, does this reflect inequality? If yes, establish equality. If no, ask what we can do to enable people so that they can take advantage of equality.

But a lot of people who style themselves as anti-racists often also focus heavily on equity as opposed to equality. Here, the process goes: you notice inequity and ask, does this reflect inequality? If yes, establish equality. But if no, directly re-establish equity. In that case, I think you have to make the case that it's better than zero-sum every single time. It may still be the right thing to do, and it may be better than zero-sum (sometimes yes, given the oft-ignored benefits of diversity), but the obviousness is lost.