Additional memory specification does not seem to consider the index for block indirect sort

Reading the paper about block indirect sort I was very puzzled of how one could manage the blocks indirectly without some sort of index, whose size would have been linear in the data (very small, like below 1%, but still depending on the data, not on the number of threads). Looking at the code, it seems to me that the variable `std::vector index` of the backbone is in fact a vector of additional `usize` values of length equal to the number of blocks. Am I correct?