Yeah, in the case of the Exit Index, the depth appears to be connected to an envelope, so when you play harder, the tremolo isn't as pronounced as when you play lighter. I haven't attempted to trace it out from the pictures Grooms/DBA posted over the last few months of the PCB, but I assume the dirt is post-tremolo so the effect is even more noticeable than it otherwise might be.
Personally, I've always had trouble with tremolos, and preferred stereo panning over conventional tremolo effects. Add to that, I already have a Slicer, so the programmed type tremolo (which I'm not particularly fond of to begin with) of the Mattoverse isn't a selling point to me, anyway.
Something I didn't even think about, though, is that the perceived envelope effect was actually just the utilization of the expression input.