That vowels are an afterthought in Semitic scripts is not an “Orientalist claim” at all, it’s completely mainstream archaeology. The use of some consonant letters to denote long vowels (as so-called matres lectionis) was a development subsequent to the use of a purely consonantal writing system.
With regard to Turkic, one of the challenges with using the Arabic script is not just that Turkic has a larger vowel inventory than Arabic or Persian, but the frontness/backness of vowels plays a major part in the morphophonemics. The Orkhon script that was the first used for Turkic languages was designed to reflect this, but when the Arabic script was introduced in towards the Middle Turkic era new strategies had to be thought up to reflect frontness/backness, e.g. the use of Arabic emphatic stops versus non-emphatic ones. However, the solutions that were found left a lot of room for ambiguity. I often read Kazan Tatar documents in the old Arabic script, and I can understand how the plethora of rules and exceptions confused the masses prior to the introduction of the Latin and Cyrillic alphabets. (The Cyrillic script now in use is hardly better than the Arabic script, though)
Then alphabets evolved from that like Latin and Greek with all the bells and whistles. Semitic scripts on the other hand remained true to its roots and didn't evolve to have dedicated vowels early on.
Arabic as a Semitic language descended from Syriac and Aramaic and thus lacked distinct vowels. Heck early Arabic script didn't make any effort to differentiate similar looking consonants as diacritic were totally missing. Case in point, Hejazi Script, one of the early Arabic scripts lacked vowels, vowel diacritics and consonant diacritics but only possessed the defining quality of cursive/adjoining writing.
However, for later versions of the Arabic script, things improved substantially esp. when it had the full support and backup of the then-young and burgeoning Islamic state, and thus it underwent a complete overhaul where it got all the bells and whistles of other alphabets but retaining a few distinctive features like compactness.
So, yeah you can say that vowels were afterthought in early Arabic scripts but definitely not for the current system in use for centuries now and that's why I characterized his/her statement as an orientalist claim that's completely inaccurate and improper.
For Ottoman Turkish, I get the frustration that some Turkish speakers may have had with reading or writing in Arabic script. Original Arabic script is not supposed to be a drop-in replacement for any language. It needs first to be extended and re-purposed to meet the requirements of the target language and with languages like Turkish with a wider selection of vowels, it gets tricky to work around the limitations of the script like vowel diacritics.
Absent these additions and workarounds, it becomes more advisable to make the switch to more accommodative script like Latin and forgo the succinctness and terseness gains of the Arabic script and that's why I view Kurdish written in Arabic script as a big mess as the developers opted to full hard code of the vowels in the script while dropping vowel diacritics altogether.
> thus it underwent a complete overhaul where it got all the bells and whistles of other alphabets
It didn’t get all the bells and whistles of other alphabets in actual practice. Yes, in theory short vowels could be denoted with diacritics, but this was rarely done in Arabic, let alone Turkic.
> not for the current system in use for centuries now
Again, the “current system” in use for centuries in Middle Turkic and early modern Kazan Tatar and other Kipchak languages did not mark most vowels with the use of diacritics in spite of their theoretic availability.
> It didn’t get all the bells and whistles of other alphabets in actual practice.
Like what exactly? What's missing of value?
> but this was rarely done in Arabic
Because it's redundant. I know it's frustrating for beginners to guess the diacritics on the words but once you get to intermediate/advanced proficiency level of the language, you'll start to appreciate this design aspect of the language.
> Again, the “current system” in use for centuries in Middle Turkic and early modern Kazan Tatar and other Kipchak languages did not mark most vowels with the use of diacritics in spite of their theoretic availability.
How's this Arabic's fault?
To be honest with you, I am not really familiar with Ottoman Turkish let alone other Turkic languages and their evolution journey but if they didn't make any use of extended vowel diacritics or worse the baseline package of Arabic, I don't know exactly how they managed to communicate using that system.
While short vowel markings are left out, as long as different words can have wildly different voicings, it is hard to claim they are redundant. Rather, the reader is simply pressed to tell the vowel pointing from context, a skill that does not come without considerably more literacy education than for alphabet writing systems. The claim that diacritics are redundant would be more easily defended for languages like Romanian where the sounds distinguished by diacritics still usually stand in an allophonic relationship dependent on the word’s morphophonemics, but that is certainly not the case in Semitic languages today.
> How's this Arabic's fault?
The Turkic script wasn’t introduced to the Turks in a vacuum. It was brought in as part of a larger influence of Islamic culture, and because among Arabic and Persian speakers the script was almost always used without short vowel diacritics, the Turks inherited the same “right way” of doing things, disastrous as it was for literacy in their languages until the early 20th century.
With regard to Turkic, one of the challenges with using the Arabic script is not just that Turkic has a larger vowel inventory than Arabic or Persian, but the frontness/backness of vowels plays a major part in the morphophonemics. The Orkhon script that was the first used for Turkic languages was designed to reflect this, but when the Arabic script was introduced in towards the Middle Turkic era new strategies had to be thought up to reflect frontness/backness, e.g. the use of Arabic emphatic stops versus non-emphatic ones. However, the solutions that were found left a lot of room for ambiguity. I often read Kazan Tatar documents in the old Arabic script, and I can understand how the plethora of rules and exceptions confused the masses prior to the introduction of the Latin and Cyrillic alphabets. (The Cyrillic script now in use is hardly better than the Arabic script, though)