Interesting. I ride MTBs, and wouldn't expect to see much left behind on hardpack, even with a heavier tire, although where a rider is positioned over the saddle or bars, whether s/he is climbing or descending, etc etc might have an impact on tread depth or angle.
Someone could also, in theory, roll rather than pitch a bike down or off a road or trail, but there would be minimal control of direction or angle. Still, that tactic look more like a "natural" fall than winging the same bike from a truck bed -- and mountain bikes are awkward, with all kinds of edges and angles and weight distribution -- never thrown one, but have spent years lifting and carrying them, and it ain't always easy, especially if you're heaving rather than toting.
Final thought. MTBers get used to falling, especially on challenging terrain, but there tend to be particular ways of falling, and using your body to prevent a fall, or a minimizing its impact, dragging a foot in a skid, say, or rounding out at the base of a jump or slope. So I wonder if LE have spent time examining possible trajectories of such a fall, with or without mountain lions, and if that will factor in.
JMO, IMO -- have been away from this case so this speculation may be completely irrelevant.