Not OP (OC? Not the person you were helping, you get what I mean), are you sure you meant df -h? fd -H seems more useful for to me when trying to find a specific file in a dotfolder, though even that didn't work on my system. fd ignores ~/.config by default, so you need to use fd -u (which is an alias for fd -I -H) to find the correct files.
Anyways, from your description it seems like the correct file would be ~/.config/kwinrc, which exists on my system.