Arabic writing can be also understood without diacritics. Was the Quran ever written without diacritics?
The oldest manuscripts of the Qur'an are only very sparsely pointed, like this one:
If they do have vowels, these are indicated by red dots above or below the letters, not with the later standard vowel signs, like this one:
(The second of these has some "modern" vowel signs added later in black ink.)