OK, I did not read the source code of the FUGC, but the article mentions "FUGC relies on a sweeping algorithm based on bitvector SIMD." So, assuming there is just one bit per block of memory, then there is no need to visit the memory of the dead objects in the sweep phase. The bit of the dead object is zero, and so that memory block is considered free and available for reallocation. There is no need to visit the free block.

It's true that it doesn't dirty up your dcache, but it's "visiting" enough that it wouldn't "need a lot more special support if it wanted to report the dead objects," which is the context of the discussion here.