Did you drive in D or L for the ascents and descents?
The big issue with this theory is the posted speed limits on roads like this tend to be 35 MPH or lower. I've seen some in Colorado where the switchbacks are so frequent and sharp the posted (white sign) speed limit is 15 MPH. On the other hand, the Volt is definitely not the car I'd want to enter in the Pikes Peak Hill Climb simply because you do have a lot of slow hard turns followed by hard acceleration. The Naturally Aspirated ICE in the Volt simply wouldn't have enough power to maintain the battery SOC and the battery is probably too small for this type of driving. On these two climbs there was enough battery SOC to accelerate from 50 to 70 MPH going up Vail Pass after passing a car that had just passed a semi and the from 50 to 80 MPH going up to the Eisenhower Tunnel after to get out of a three lane jam of slow cars. In both cases I resumed the cruise control and let my car slow back down - the Colorado State Police actually monitor the three lane climb to the Eisenhower Tunnel for aggressive driving and I actually did see someone pulled over about two thirds of the way up to the Tunnels.
I had a similar thought driving across the high desert valley between the east end of Glenwood Canyon (6,000 ft) and Avon (7,400 ft) at the posted 75 MPH. In fact, based on prior experience in Kansas at 75 MPH where Mountain Mode was 100% depleted I wasn't sure the car would maintain the "empty" SOC in preparation for the actual Vail Pass climb that starts only a few miles east of Avon (Avon is west of Vail). Battery's SOC for "empty" was apparently maintained and I had sufficient battery power on both approaches to accelerate between 20 and 30 MPH. The battery portion of the power meter reached 100KW on both those accelerations.
Based on the power differences I've seen when I'm alone in my Volt vs. when my wife and I are travelling together with a fully load cargo area/back seat I suspect the ICE is still powerful enough by itself to not require Mountain Mode. Remember, the Volt's ICE is Naturally Aspirated (NA) and I took the car up the steep approaches to the two highest points on the US interstate system at 65 MPH and it didn't miss a beat. The ICE screamed but didn't miss a beat and wasn't maxed out on the power meter