This last paragraph before the review seems relatively ambiguous, although I may be too confused to grasp the thrust of it. Is the premise we made up of B→A just a common pitfall that people make in translating the given scenario, or is it actually a correct translation but wrong in the larger context of the previous example?

Is the second argument, which uses the term only, logically distinct from the first argument? If it is logically distinct, then how is the premise still A→B. If not, then I don't see how the use of the term only doesn't change the premise to B→A. It seems valid for there to be some member of the set of birds which doesn't migrate south in the winter, but it would not be valid to say that there is a member of the set of not-birds which migrates south in the winter. Therefore B is a subset of A. With that, membership in the set of animals that migrate south for the winter is sufficient to guarantee that said element must be a bird. I.e. B→A.

I hope that my question(s) make sense, please let me know if I can clarify anything.