Nope. Doesn't have that kind of transmission eg. NO torque converter but more like a differential in reverse. Sort of. It's a PSD (Power Split Device) and actually can't "slip".
I vote for the steep incline "confusing" the VSC system somehow.
On a side note my Prius (and current gen Priuses but have not tried it with my TCH) can exhibit a similar behavior on a steep incline with loose gravel. If the front wheels begin to slip the VSC reduces power to the wheels. If the wheels continue to slip it reduces power more. Right down to ZERO power to the front wheels and even with the "pedal to the metal" you go nowhere and the wheels won't turn.