Rotorspeed - the only thing we can be sure of is that they crashed into the hillside and therefore you can be pretty sure it wasn't visible to them.
I agree it probably isn't a LOC IMC, more likely not flying high enough but I don't think the zero degree isotherm was that low, IIRC it was around 6 -8000' that day.
The weather in the area was lots of stratus with bases around 3 -400' - he may have encountered this some time earlier and elected to climb IMC, hoping for cloudbreak the other side of the hills.
Whatever the reason, he just wasn't high enough for the terrain to be flying IMC.