It's AVX512 that makes the gains, not assembly. This kernel is simple enough that it wouldn't be measurably faster than C with AVX512 intrinsics.

And it's 100x because a) min/max have single instructions in SIMD vs cmp+cmov in scalar and b) it's operating in u8 precision so each AVX512 instruction does 64x min/max. So unlike the unoptimized scalar that has a throughput under 1 byte per cycle, the AVX512 version can saturate L1 and L2 bandwidth. (128B and 64B per cycle on Zen 5.)

But, this kernel is operating on an entire frame; if you have to go to L3 because it's more than a megapixel then the gain should halve (depending on CPU, but assuming Zen 5), and the gain decreases even more if the frame isn't resident in L3.