That's not even remotely correct for this one, although some of the samples do come from a soundfont. Just that they're not a SNES soundfont at all (as in, they're not ripped from a SNES game)...
And with regards to making volume slides more smooth? Under less extreme circumstances, I would have envelopes be processed in half-tick intervals (or even quarter-tick, but that could be pushing it for slowdown-free playback). Under more extreme circumstances, I would increment/decrement the direct gain while waiting for the next tick. A side effect of this would be the ability to go even lower in tempo since now the lower tempos can actually fit in an 8-bit number due to the increased precision.