This is the best solution that they’ve been able to find for FSRS, because there’s no way to know if the interval created by Set Due Date is “real” or not. [It was touched on briefly here, but the longer conversation is on Discord, and a few different places in GitHub.]
When you’re using it to scatter a set of cards across the next few years, you’re not using any criteria to determine the interval. So you don’t necessarily know the card set for 500d better than the card set for 100d. FSRS can’t rely on that interval at all for setting the memory state.