Look, I take what people say at face value if there is no compelling reason not to. The Mandaeans call themselves gnostics, which is what their endonym means. They also use a form of Eastern Aramaic as their liturgical tongue, so their origin likely lies prior to the Arab conquests or Sassanian Persia. The dominant exonym used by their Arab neigbours is some form of Sabian: This is either related to their baptising practices, or an association made with the Sabians mentioned in the Quran. Either way, it doesn't point us to an alternate origin from that implied by their own name as Gnostics.
So I am fairly sure they are the last surviving group of the ancient Gnostic religions. Like all gnostic groups, they were fairly syncretic, so we see them adopt certain prophets or not, as they see fit. It is also possible that they have some relation to Manichaeism, but I don't think that can be shown conclusively - if you could, then arguing them the Elcaisites would be easier, as Mani likely descended from that sect.
A more interesting question is if they are an offshoot of an esoteric Jewish school, like the Essenes or even from followers of John the Baptist. I find the idea that Gnosticism as a practice predates Christian belief fairly convincing, so I think syncreticism more likely for it and Mandaeans. Historic Gnosticism is too odd, too off from either Jewish or Christian practice, and too established therein, for me to think it such an early corruption rather than adoption of figures (akin therefore to something like Sihkism or Baha'i that co-opts other faith's prophets). The fact that heavenly pairing, divine syzygies with demiurges and fallen worlds of light, are present therein and in other gnostic schools like Valentinianism, I find convincing. This can be clearly paired with Hellenistic thought, like Orphism's Katabasis or asccending through heavenly spheres, so a Jewish origin is doubtful.