Borusyak, 2021 has a sentence

We show that this estimator takes a particularly transparent form under unrestricted treatment effect heterogeneity, while our construction also yields efficiency when some restrictions on treatment effects are imposed.
I am wondering what does under unrestricted treatment effect heterogeneity mean? I translate it to that "when the treatment effect heterogeneity is not restricted", and it also means that this situation works in both cases "treatment effect heterogeneity" and "treatment effect homogeneity". Is it a correct thought?