Do you feel like your running pace and HR threshold values are set properly? It sounds to me like maybe your running threshold pace values are set too low—that’s a very high rTSS for a one hour walk that can’t have covered much distance.

I’m an ex nordic racer, current skimo racer and ultra trail runner.
all XC skiing is not the same:

you can shuffle along at an effort similar to walking.
you can kick-through and glide in your classic skiing technique while pushing hard with arms.
you can also skate ski.

your comparison of XC skiing to walking really depends on how you are skiing.

I have been using Training Peaks Premium for close to 4 years now and don’t pay any attention to the Training Stress Scores anymore.

The training stress scores appears to be time driven with a factor for activity type. This is fine for time and distance sports like road running, cycling and lane swimming but mountain sports have more variables and if you recreate in the mountains regularly your week to week plans probably depend on conditions which further complicates the process of creating an objective apples to apples stress score.

If you really want to use the stress score you could log all your activities using the HRTSS and modify it using your own factor system or commit to doing the same set of activities for a few weeks to create your own baseline stress score using the Training Peaks metrics to use going forward.

To measure training in the mountain objectivity week to week, I progress volume using time and try my best to pick objectives that have a certain elevation gain per kilometer profile. I have found recovery days are good for introducing variety because all that matters is keeping it easy and not feeling sore the next day.