It's not really very good writing - even a native speaker might stumble over the convoluted structure. The difficulty in identifying figurative "pith" within a figurative "warren" strikes me as perilously close to a mixed metaphor, and the two different usages of "point" are potentially confusing and/or clumsy. But...
...his [essay] is a warren
the author likens the structure of the essay to a (rabbit) warren (a dense network of interconnecting "threads, arguments, ideas")
hard for ... us to pin point
the verb pinpoint (which should be written as a single word) can be defined as find or identify with great accuracy or precision. It metaphorically alludes to the sharp point of a pin representing a very precise position, and to the idea of "capturing" something (such as a butterfly) by attaching it to a surface with a pin.
the exact pith of what he is trying to say
the "literal" pith here is spongy cellular tissue in the stems and branches of many higher plants, metaphorically representing the essence, central point of what the essayist is saying.
that ... is as definite an encirclement [of his main point] as we could ever get
to encircle is to form a circle around, surround. The author likens the essayist's primary argument (what he has to say) to a point (defn 4.1 there), which at least partly alludes to the sense of to point = to indicate [the direction or destination of his argument]. In this context, the author means what he's just written earlier encapsulates, surrounds, defines the boundaries of the essayist's argument as well as is possible. The clause is thus an extended spatial figurative usage wherein the essayist's particular (point-like) thoughts are located within an extended metaphoric space, and the author is trying to tell us exactly where in that "space" those thoughts are to be found.