I also took my car through the mountains last week, in this case Western Maryland and Pennsylvania. I also was seeing the power meter at 12 o'clock, and occasionally one major mark past it, to maintain 75 mph uphill. From another thread on here, I believe the 12 o'clock position represents about 90 hp. So, it would only make sense that if the ICE is trying to replace all the power being used, and the ICE has barely over 100 hp peak rating, that it will be operating at high RPM to produce 90+ hp.
My new preferred technique for this driving is to switch to EV mode at the base of each hill, as the battery can easily produce the same 90+ hp without the noise, then switch back to HV at the top (where it stays in EV anyway) and regen and use HV until the next hill. Since the eastern hills are around 1000 ft individually (but often steeper than western roads), I only lost around 5 mi range on each while getting some back on the regen. I don't care about losing EV range as my goal is to arrive with no EV range anyway, and if it gets too low for comfort I can use HV Charge.
OK, geek time: a 4200 lb car will gain about 1 kWh of energy by being raised 650 vertical feet. Which means, assuming around 4 mi/kWh, that it will lose 1 mile of range for each 150-200 vertical feet. So at a 3-4% grade, for each mile of horizontal distance, you will also lose another mile of range from elevation, costing 2 miles for each one forward. At 6-8% it will be 3 miles for each one forward.
At 75 mph, which is 110 ft/s, on a 5% grade it will ascend 5.5 ft/s which will require 31 kW = 42 hp, which doesn't account for drivetrain losses or the power already needed to maintain speed against air resistance (probably greater than 25 hp). So, my observation of 90 hp seems to be within reason.
If you see anywhere I messed up the unit conversions, comments are welcome.