He finds that the prose rhythms of the first stanza contrast strikingly with the metrical regularity of the quoted line about the belle undressing. That line is so delicately honest that it almost had to be quoted in order to give the speaker some distance from it.
He notices that the prose rhythms of the first stanza are markedly different from the metrical regularity of the quoted line describing the belle undressing. This line is so subtly truthful that it seems necessary to quote it to maintain a certain distance for the speaker.