That’s the vim antifeature - the .swp file in the same directory in which is the file being edited. Not all directories are writable, but files in them might. And this is the case with /sys filesystem.
Try updating the file this way, as root:
echo 90 > /sys/class/power_supply/BAT0/charge_control_start_threshold
(or any other number then 90, to your liking).