There is another effect that is hard to capture.

During a steady sate effort, say a 10 mile long hill climb, then the speed may be close to the 10% deficit for a hard climb. Still, the heavier person is working for 10% longer.

However, many hills are much shorter, and can be conquered with anaerobic power.

So, say one can ride at a steady state of 100 watts.
Then hits the hill...

Say one budgets anaerobic energy of 150 additional watts (250 total) for 30 seconds, or 4500 watt seconds.

The light person gets to the top and starts cruising back down the other side while the heavy person is still climbing the hill, and either burns out at the 30 second point, or powers down allowing them to extend their energy budget.

So, my guess is they in fact, may ride slower than otherwise predicted.