The first thing that is probably giving you trouble is the construction concerning itself as it does. This is an instance of the general construction VERBing as SUBJECT DOtense,person, which is a way of saying since SUBJECT VERBtense,person (so much). Thus:
Disliking Bob as I do I will not vote on his promotion. = Since I dislike Bob so much I will not vote on his promotion.
Concerning itself as it does with ... = Since it concerns itself with ...
The other piece that may confuse you is speaking truth to power. Rosenbaum here uses the word power metonymically to represent the ironically capitalized entities ‘Authority’ and ‘the State’ in Auden's poem.
The sentence may be paraphrased, then:
The stanza in which the line occurs is admittedly not his best, since it shows Auden congratulating himself on bravely telling the powers-that-be a truth they do not want to hear.