It seems there are different of opinions whether Mandeans just are the Sabians, or another group. Quoting wikipedia:
Many scholars have identified the Sabians to be the Mandaeans
Question: Are Mandeans considered Ahl-ul Kitab?
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As much as I research, I concluded (based on my observation) that: firstly, Mandaeans is another name for Sabians secondly, as you yourself mentioned in your query, apparently there are diverse viewpoint regarding them if they are Ahlul-Ketab or not. Namely, a number of scholars don't consider them as Ahlul-Ketab, but there are other scholars who regard them as Ahlul-Ketab. For instance, based on my research, I found that:
Ayatollah Khamenei as a famous Shia scholar/Marja'al-Taqlid consider them as Ahlul-Ketab.
As some related informative issue concerning them:
they have a book called Suhuf Adam (Books of Adam) and Ginza Rba. They believe that the prophet John (Yahya) conveyed the book to them 2000 years ago in its current form as a transmission from earlier prophets, Adam (a), Noah (a), and Seth (a). The Quran has addressed them along with the Jews and the Nazarites as People of the Book and believers in God. However, there are doubts about whether today's Sabi'un, who are also called Mughassila and Mandaeans and live in southwestern parts of Iran and southeastern parts of Iraq around Tigris and Karkheh rivers, are the same as the ones referred to in the Quran.
And so forth.
For further information, you can refer to the following source: