In an unmodified TR-808, accent uses a higher amplitude pulse to trigger the sounds. The trigger voltage without accent is 5V and the global accent can be adjusted between 5 and 15V.
UnPulse uses the same trigger shape and voltages. However, here velocity controls the pulse voltage and thus you get full control over each instrument independently. So it behaves like the original accent but with more control and per-instrument.
So to answer your question about timbre change: Yes, it is of course still there. If you send a note with full velocity, you will here the same sound as with accent and the accent knob adjusted to full. If you send a lower velocity value, it is like turning down the accent amount knob.
As an added bonus, you can actually trigger with lower volumes as well which creates sounds that can not be played with the unmodified instrument. As you decrease the Snare trigger voltage below 5V, the “Snare” part of the sound starts to disappear leaving only the “drum” part. – Just like with a real Snare!