The question is:
To me, after one concludes that a thing cannot cause itself into
being, one could instead just argue that something else causes the
universe and that is God, but instead he brought in this "Plotinian
discussion of unity and multiplicity". What is this discussion and how
does it help Al-Kindi's argument?
After al-Kindi argued that the universe had a beginning and that it could not cause itself to be, he wanted to show how God could have made the universe from nothing. His view of God guided how he attempted to do that.
He was aware of the Neo-Platonic True One presented by Plotinus, but Plotinus claimed the universe was eternal. (Enneads, II.1.1.) If God were like the True One he might be able to use some of what Plotinus did to show how the True One related to the multiplicity of the material universe viewing it however as having a beginning.
Al-Kindi was also sympathetic to the theological positions of the Mu‘tazilites. See Adamson, Peter, "Al-Kindi", The Stanford Encyclopedia of Philosophy (Summer 2018 Edition), Edward N. Zalta (ed.), forthcoming URL = https://plato.stanford.edu/archives/sum2018/entries/al-kindi/ for more information. In particular:
...al-Kindi held an austere view on the question of attributes, on the
basis that predication invariably implies multiplicity, whereas God is
unrestrictedly one. This has been compared (Ivry 1974, Adamson 2003)
to the position of the Mu‘tazilites, who were the main contemporary
theologians of the ninth century.
Although Craig traced in detail al-Kindi’s argument when it came to the universe having a beginning and not being self-caused, he felt al-Kindi’s view of the True One involved a “rigorous determinism” (*The Kalam Cosmological Argument, page 35). Based on his reference to al-Ghazali in Theism, Atheism, and Big Bang Cosmology, page 66) he apparently preferred a view of God as a personal agent:
The answer to Kant’s conundrum was carefully explained by al-Ghazali
and enshrined in the Islamic principle of determination. According to
that principle, when two different states of affairs are equally
possible and one results, this realization of one rather than the
other must be the result of the action of a personal agent who freely
chooses one rather than the other.
So, after showing the universe had a beginning and did not create itself, al-Kindi needed to show how God could create the universe from nothing. He used Plotinus’s True One as his view of God. This was in accord with the views of the Mu‘tazilites although it involved a “rigorous determinism”. Craig preferred a different argument provided by al-Ghazali that viewed God as a personal agent.