Amiga has a 14-bit (or more) DAC.
You are confusing the sample size with the DAC size. The samples are sadly, only 8-bits, but they happily get 6 bits of volume control added to them for a total of 14 bits that the DAC outputs.
If the DAC was only 8-bits then the volume control registers would have no effect and all Amiga mods would sound like crap.