Hebrew, Syriac and Arabic all use their letters as figures, and Mandaic descends from the same Aramaic stock, so the practice would be unremarkable. No alphanumeric numeral system for Mandaic is described in the reference literature consulted, and the modern character standard encodes no Mandaic digits. Nothing states that the practice was absent either, so this is a gap rather than a disagreement.
What would resolve this 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.
OpenSemitic palaeography and Mandaean manuscript studiesThe Unicode Consortium, The Unicode Standard