Hebrew, Syriac and Arabic all use their letters as figures. The system is inherited with the sequence: units, then tens, then hundreds, so that the letter at the nineteenth place opens the hundreds in each of them. Mandaic descends from the same Aramaic stock and would be unremarkable for doing the same.
No alphanumeric numeral system for Mandaic is described in the reference literature consulted for this atlas, and the modern character standard encodes no Mandaic digits. That is the whole of the evidence, and it is evidence of silence rather than of absence. Nothing anywhere states that the Mandaic letters were never used as figures.
So the atlas records this as unknown rather than as a fact either way, and carries an open question saying what would settle it. What would settle it is a survey of numbering in dated Mandaean manuscripts and inscribed lead rolls, checking colophons, quire marks and folio numbers for letters standing as figures, together with a reading of the standard Mandaic grammars on how numbers are written out. The disciplines that would have to do it are Semitic palaeography and Mandaean manuscript studies.
This is the atlas doing what a reference work usually cannot. Most books either assert that Mandaic has no numerals or say nothing at all. Naming the gap, saying what would close it, and saying who could close it leaves the door open for someone to walk through.