The YM2149F has 3 channels that you can output a 4-bit logarithmic value on. The naive implementation would be 3 x 4-bit samples, yes, and if you didn't compensate for the log curve the samples would be pretty distorted.
However, it didn't have to sound bad/distorted, and it didn't have to be limited to 3 channels. You can instead do your mixing in software and use the channels as a single linked DAC, allowing more than just 3 channels and better sample quality. It requires a bit more clever programming, but the results are pretty decent.
There was a really decent sounding 4-channel tracker for the ST called Quartet. I think it's output was about equivalent to 8 bit / 16kHz. Example: https://www.youtube.com/watch?v=SPhcNthcweQ