The claim that dad is unique to Arabic rests on what the letter sounds like, and what it sounded like when the claim was first made is not the same as what it sounds like now. In modern standard pronunciation dad is an emphatic d, articulated like dal with the back of the tongue raised. That is not the sound the early grammarians describe.
Sibawayh, the eighth century grammarian whose account of Arabic phonetics is the earliest systematic one to survive, places dad at the side of the tongue against the molars and describes an articulation that later readers have taken to mean a lateral sound, something closer to a sideways emphatic l than to a d. Several lines of evidence have been brought to bear: the descriptions in the grammatical tradition, the way Arabic words containing dad were borrowed into Spanish, Persian, Turkish, Malay and Hausa, the pronunciation preserved in some peripheral dialects of southern Arabia, and the treatment of the letter in the recitation manuals. They do not all point the same way, and reconstructions of the original value differ.
This is a genuine scholarly disagreement rather than a gap: the evidence is public, several reconstructions have been argued at length, and specialists still hold different positions. The atlas records that the disagreement exists and does not choose between the readings. What is not in dispute is that the sound changed, that the change happened early enough for the recitation tradition to have carried the older value for a time, and that the identity claim attached to the letter outlived the sound that prompted it.