The translations here are incorrect.
Funga should mean to score a goal. Fungwa == Concede a goal.
Shinda means win, Shindwa means lose/be defeated.
You can concede a goal but not be defeated. So for this sentence "fungwa" and "defeated" mean different things.
"wamefungwa bao moja" = "they have conceded one goal"
"they have been defeated by one goal" = "wameshindwa kwa bao moja"
I think it's fine as it is, and I think if you add na it means literally that it was the goal that beat them, not the other team. I think you could possibly add kwa here to make it clear that "one goal" is essentially the tool with which / degree to which they were defeated ... but don't take me as an authority here.