[jhj]

This is product quantization (a vector is chopped up into sub-vectors where each sub-vector is quantized using vector quantization (VQ)), not scalar quantization (which is what you're comparing it to here).

Also most scalar quantization methods use uniform quantization (e.g., divide the range between the scalar lower bound L and scalar upper bound H into N different regions where N is usually 2^bit_width), whereas PQ (and VQ) is learned quantization via k-means on some training vector set, so they're not really directly comparable.