The first sentence is ambiguous as to the state of the war. Just from it, I would not agree with the assumption that the war has passed.
Mostly likely their thinking is that if the war were ongoing, they would hear:
The country is being ravaged by the war.
But I've heard numerous news reports about ongoing wars where has been ravaged is used. Just because the war isn't over doesn't mean that the country hasn't already been ravaged. (Or that it can't be ravaged more.)
Since people watching the news are aware of the status of the war, there is no reason to be more explicit.
The second sentence is just as ambiguous as the first one, but there's a key phrase that likely indicates why an assumption is being made about it being ongoing:
Yemen has been ravaged by a three-year civil conflict that has the country fighting a growing cholera epidemic.
However, all this really says is that there's an ongoing cholera epidemic. The war itself could well have stopped some time ago, and the country is simply dealing with its consequences.
But the addition of that phrase in the second sentence could explain why a different assumption is being made.
In short, there is no difference between the has been ravaged in the two sentences. It's simply a matter of assumptions (rightly or wrongly) that are being made.