When lapping a stone, the 600grit does not remove particles in the size of 600 grit, or even 2k grit. Lapping is not like woodcarving with a chisel that removes big pieces of wood. Stone for our case. You can use 30 grit sandpaper, and what you see is powder from a stone, fine powder for the fine stones. DMT says that if you use its finer diamond plates for lapping, they get worn out pretty quickly, especially in the presence of slurry, and those diamond-nickel(?) particles are also released from the diamond plate with the slurry of the stone.
Basically, many hones, especially the hard ones take forever to produce slurry from rubbing them with a hard nagura but with a diamond plate it's very easy. I don't usually prefer diamond plates for producing slurry, but how I think it works.