Who is Augustine arguing against in his Confessions, book 12?

Augustine was speaking against the ideas of Marcion. According to Marcion, the title God (Yahweh) was given to the Demiurge, who was to be sharply distinguished from the higher Good God (El - See Gen:1:1 and elsewhere). The former was díkaios, severely just, the latter agathós, or loving-kind; the former was the "god of this world" (2 Corinthians 4:4), the God of the Old Testament, the latter the true God of the New Testament. Christ, though in reality the Son of the Good God, pretended to be the Messiah of the Demiurge, the better to spread the truth concerning His heavenly Father. The true believer in Christ entered into God's kingdom, the unbeliever remained forever the slave of the Demiurge.

El was the creator of everything: the demiurge (Yahweh) was simply the god who fashioned the earth.

In his commentary on Genesis, Augustine cautions the Christians of his day from engaging in scholarly discussions regarding scientia (i.e., knowledge) in fields of study where they have no experience, lest they embarrass their religion. Genesis should not be used, in other words, to argue about medicine, agriculture, astronomy or other areas where the moors were experts.

He might be talking to the same, general group in Confessions, those who, "honour your holy scripture written by that holy man Moses and agree with us that we should follow its supreme authority" (XII.xvi) That would be consistent with his interest to make a literal translation, one that represents what the author intended. The author of Genesis was interested in spiritual, not technological, knowledge.

My translation of Confessions doesn't have chapters in the books so it was a bit hard to find this. (using Nelson's Royal Classics edition)

I beleive he is referring to other interpreters of the Pentatuech.

But in the midst of so many truths which occur to the interpreters of these words.

I think this is meant as a hypothetical intepreter.

Later on the part you quoted, St. Augustine refers to a people who pluck words out of a thicket to justify their own ends.

They see these things and they rejoice in the light of Thy truth to whatever degree they can

To me, as a Catholic in what I belive to be the tradition of St. Augustine, this sounds like a very generic heresy. (whoops alsmost said St.) G.K. Chesterton always described heretics as people stuck on a particular truth.

The intro to my translation of confessions also says that Augustine went up against Donatists and (full-fledged) Pelegians. The movie about St. Augustine, Restless Heart also shows his run-ins with the Donatists, who really seemed to have a good hold on what was then the official Church, but they disdained the organizational structure of the Church.

However, it doesn't sound much like Augustine has disdain for these hypothetical interpretations, and in the intro to Chapter Twelve the editor writes:

He emphasizes the importance of tolerance where there are plural options, and confidence where basic Christian faith is concerned.

So, it would seem as though, Augustine would prefer people to stay "in tge nest" but if they stray and pluck from the thicket (i.e. Sacred Scriptural Truths)

the footnote says that “they" which in my book is translated "one of these men" refers to the "Thicket denizens mentioned above" which he later goes on to point out are 4 distinct viewpoints, making up the notes whuch must be played together, to make up the real song of creation.

For, as Chesterton says, the Christian when confronted with two seemingly contradictory truths simply takes the two truths with the contradiction and adds it to the deposit of faith.

So these are simply men Augustine uses to illustrate one point about what might later be coined as "senses of scripture". If he took the time to take any of their suppositions to their logical ends, he'd probably find some new heresy to squash, but that's not the goal of this book.