Set a Mandaic alphabet beside a Syriac one and almost nothing matches. The shapes are unlike, the names are unlike, the numeral values are absent on one side, and the phonology has moved. What matches is the order.
Position by position, the first twenty-two Mandaic letters answer to the first twenty-two Syriac letters, and to the twenty-two Hebrew letters, and to the twenty-two places of the Arabic abjad reckoning. Az is seventh in all of them. That is not coincidence and it is not convergence: an order is an arbitrary thing, and an arbitrary thing shared by four alphabets is inheritance.
This is why the atlas links these letters to one another on place rather than on anything else, and says so in each relation. It is also why the question of where the Mandaic letterforms came from can stay open without unsettling anything. Published accounts assign the shapes either to a cursive form of Aramaic or to the Parthian chancery script; that argument concerns the drawing. The sequence is not part of it.
For a reader the practical value is that a Mandaic alphabet can be mapped onto a known one letter by letter without recognising a single shape. Someone who knows Hebrew can find the Mandaic letter that answers to samekh by counting to fifteen. That is a real piece of access to a script very few people outside its community can read.