This passage is adapted from a review of a 1991 book.
Topic
Garcia's study
Framework
Challenge a Position
Main Point
Garcia's study is cool but suffers from two flaws: it semi-contradicts itself when arguing about the diversity of views among the Mexican American activists, and it possibly assumes that the political consciousness of the activists was more widespread among the rest of the Mexican American ethnic population than it really was.
Summary
P1 - Intro of Garcia's book, a compliment, a critiquing thesis
The author establishes what the thesis of Garcia's book is and drops a little complimentary juice by saying that Garcia does provide "persuasive evidence "that the Mexican American activists in the 30s-60s were important harbingers of more famous activism that occurred in the 60s.
But we get our Purpose Pivot (but, yet, however, recently) in the final sentence. We can tell this sentence is the author's thesis because it says that Garcia's study suffers from two flaws, and then we can see that paragraphs 2 and 3 are just unpacking that thesis.
P2 - First flaw ... support for 'diversity' undermines itself
The author is basically just saying that Garcia did himself a disservice by acknowledging how much common ground there was among the Mexican American activists of the 30s to 60s, given that Garcia's main point was to convey the diversity of thought within this group (and given that there definitely WAS a lot of diversity within this group).
This is a weird complaint, because it's not the author disagreeing with the content of Garcia's thesis; she's just complaining that he did a poor job in executing his supporting evidence, since he emphasized togetherness, at the expense of better communicating diversity.
P3 - Second flaw - author thinks activists were an unrepresentative sample
Here, the author is basically saying that Garcia may go too far in thinking that the political engagement of the activists somehow reflected broader trends among the Mexican American population, when the author feels like most of the regular Mexican Americans were still pretty culturally conservative and shut-off from mainstream American politics.