Indeed this letter stood for /ʃ/ in Aramaic and one must ask why 𐢖 from ܣ from ס from 𐡎 from 𓊽 has been lost in the Arabic alphabet.
One must mind the three Proto-Semitic sibilants: ś, š, s. In Arabic the first gave /ʃ/, the second and third merged to /s/. In Old Aramaic all were preserved but the the orthography represented s by 𐢖 / ס / 𐡎 but used 𐢝 / ש / 𐡔 for both ś and š, and this orthography persisted even when by the time of the Nabataeans ś had merged to sound as /s/ as s always did.
So when a scribe wanted to write the Arabic sound /ʃ/ from ś, he did so by using 𐢝 / س to stick to a letter form that looks the same in the cognate Aramaic words. But when he wanted to write an Arabic sound /s/ from Proto-Semitic š, he would have defied the Aramaic spelling for this Proto-Semitic phoneme had he written 𐢖 / ס / 𐡎, whereas Arabic-Aramaic word pairs with the Proto-Semitic s are rare. So the impression was that 𐢝 / ש / 𐡔 / س is the most typical sign to write /s/.
Diem, Werner (1980), “Untersuchungen zur frühen Geschichte der arabischen Orthographie: II. Die Schreibung der Konsonanten”, in Orientalia (in de), volume 49, issue 1, DOI:10.2307/43075525, pages 77–81