One could argue that only having 18 attacks for the week might also cause war losses :^P the other player, albeit having lost, has participated more. 18-5 is quite poor though, ha, sounds like he could use some advice.
This is why against smaller fields some players will purposely lose a battle or 2 (by sending in a single low hero, or retreating immediately) - so they are being fair by not taking all their hits, but still getting some points for showing up.
I do agree it's a little silly & would prefer 0 points for a loss, but it's just how it is & always has been. War has long been in need of an overhaul that's never materialised