>> Herodotus described a group of cave-dwelling Ethiopians. “Their speech is like no other in the world: it is like the squeaking of bats,” he wrote. We can’t know for sure which communities he was describing, but Meyer says that several whistled languages can still be heard in Ethiopia’s Omo Valley.
In Herodotus' writings, like in many other ancient sources, the name "Aethiopian" (and not Ethiopian) was used for people from Sub-Saharan Africa:
Considering that the word barbarian comes to us from Greeks describing any language that wasn't Greek as the like the baying of sheep I wouldn't take "bat-like squeaks" too literally.
Maybe he didn't embellish his stories, but he certainly wrote down a lot of things that were clearly folk tales and hearsay and of which he had no way to check the veracity. You can't really make anything of his reporting of "Aethiopian bat squeaks".
Oh and btw- he was not the only (ancient) Greek interested in a more rigorous look at history. You have probably heard of Thucidides, Xenofon etc.
Whistles can be a fun way to control a system, the DSP needed to do this is really simple, and whistles really punch through the noise floor like nothing else!
I've always used discrete change in pitch as the "symbol", so commands are encoded as (for example) UP UP DOWN UP, so this only depends on the relative pitch change between short whistles.
The article does give an example of how it works in Spanish. There is an audio sample linked of the sentence 'En todo el mundo hay hombres que hablan silbando' being whistled.
In Herodotus' writings, like in many other ancient sources, the name "Aethiopian" (and not Ethiopian) was used for people from Sub-Saharan Africa:
https://en.wikipedia.org/wiki/Aethiopia
If people in Ethiopia have a whistled language it has probably nothing to do with the "bat-like squeaks" mentioned in the article.