Drum brakes are one of the many things that contribute towards EVs becoming lighter than equivalent ICE cars over time, as they weigh less than disc brakes (about 25% lighter, 10lbs maybe, and unsprung weight too).

As the link states, they also increase efficiency, and that allows for smaller batteries, less weight. They're also looking into lighter weight aluminium disc brakes because EVs use them less, and it prevents issues with low usage disc brakes rusting.

I think the idea is that engine braking (regenerative braking) makes up for the difference.

Can you rely on that if someone charges their car at elevation and then has to drive down a mountain?

Yes, by not charging to 100%, so you have some room for additional charging by regenerative braking. Virtually all EVs come with a setting, usually enabled by default, that stops the charge at a preset percentage, like 80 or 90%. Chevy aptly calls theirs "hilltop reserve".

My PHEV weighs 30% more than the non-PHEV version, yet my brakes have lasted longer than anyone I read online with the regular version of my car.

This is because of regenerative braking not using the brake pads, using the motor instead.