If I had to guess the primary basis for his F1 rating is the damage it caused at the scout camp; tools like OpenMapTiles and Google Maps satellite view obviously didn't exist when SigTor was first published, and with the sheer amount of tornadoes to cover (along with the lack of detailed survey information, as opposed to the Teton-Yellowstone F4) I doubt he was able to devote much attention to a tornado that caused very little damage to man-made structures.
With that said, the totality and severity of the tree damage this thing caused is nothing short of incredible; if you would like to check it out for yourself: punch these coordinates into Google Maps or the like:
40.673868, -110.120258
Though listed as three separate touchdowns I believe there were two separate tornadoes; a first, brief touchdown followed by a longer path consisting two areas of felled trees. The likely reason it's listed this way is because the second tornado crossed over an area with absolutely no DI's for a time, and the possibility of the cell cycling during that time isn't out of the question, though the paths can be easily connected and a weakening trend identified as such. The coordinates for the second area of blowdown from the second tornado:
40.711483, -110.051636
Coordinates for what I believe might be the first tornado: not 100% sure though, the Tornado Talk article had the exact coordinates but naturally it's behind a paywall. Oh well I guess?
40.625493, -110.202910
It's also amazing just how well preserved the damage from this tornado is; it's probably one of, if not the best example of preserved tornado damage in the world, and shows that intense to violent tornadoes can alter the landscape for decades if not centuries.