FWIW, I geotag the photos I take with my “real” camera using this method, and so far I haven’t been able to find any geotagging program that automatically uses horizontal accuracy from a GPX file (if that’s even recorded?). Thus, about 19k of my observations lack horizontal accuracy data.
If you go to the observers list of your search I think pretty much all of the top observers - outside of Questagame - use dedicated cameras and likely have similar workflows to me, or have cameras with built-in GPS that don’t record accuracy data.
EDIT: I’ll also add that I use a Garmin eTrex 20 and when I check horizontal accuracy while out on a hike, it’s usually in the 2-5m range, depending on topography and cover.