Yes, hiking is generally less intensive than running. It doesn't matter how steep you go, it matters how fast you go.
On the other hand, optical sensors are not perfect, and from my experience, they work very bad on low intensity exercises. I don't know if it's an energy conservation mode when you're on low intensity, that is turn off by running, or it's a sweat that block the sensor, but I experience often that my Garmin fails to detect higher heart rate while hiking. Maybe because by running, I try to keep the watch open, so that the sweat doesn't accumulate so heavy, or it's just that higher pace and higher HR keep the energy save mode off...
Don't forget that sport watches are optimized for sports, and other options are added to do extra sales, so it's fully plausible that Garmin engineers have tested and optimized for runs and cycles, but not for walking and hiking.