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 discerns that the prose rhythms in the first stanza sharply contrast with the metrical consistency of the quoted line about the belle undressing. The line's subtle honesty necessitates its inclusion to provide the speaker some separation from it.