I always wonder if this "no one wrote down vowels" thing was really because these languages were spoken without vowels. The insertion could just be projection of the current situation onto the past. I mean like a language consisting of words like this one:
I always wonder if this "no one wrote down vowels" thing was really because these languages were spoken without vowels.
The issue with your theory is the numerous counterexamples of languages from the same region which did not (and in fact still do not in plenty of cases) include vowel indicators in their writing systems, but nonetheless spoke and still speak the vowels.
Also, Egyptian is known to have included "determinative" hieroglyphs for cases where the reading would otherwise be ambiguous; the determinative allowed words with the same consonant structure but different vowels to be distinguished from each other (though the determinative itself did not encode a vowel; instead it indicated what type or category of thing the preceding word was, which allowed the reader to choose the correct option).
Plus your example of a vowel-less word is from a language which does have vowels:
A lot of Afro-Asiatic languages like Arabic and Semitic also have the "write no vowels" pattern. That being said, there are other Afro-Asiatic languages like Berber that have those long consonant strings. I am not too familiar with these languages, but I wonder if there is some schwa epenthesis happening in those clusters.
xłp̓x̣ʷłtłpłłskʷc̓ https://en.wiktionary.org/wiki/x%C5%82p%CC%93x%CC%A3%CA%B7%C...