and all without mistaking any thing for what it may not be (16-18)
Every single "thing" in poetry counts. And it's the poet's responsibility to perceive it as it is, rather than what it's not. But is this really possible? Can anyone be so absolute about "any thing?"
For he's the super realist who must perforce perceive taut truth (19-21)
He's a superhero realist. Sounds kind of anticlimactic, right? Then again, if there were ever a person who could really do such a thing, the world would probably make more sense to the rest of us.
in his supposed advance toward that still higher perch (23-24)
Truth is also up there with Beauty, right? The two may be one and the same. But still, it's only a "supposed" advance. So even if the poet thinks he perceives "taut truth," there's no quiz at the end of the wire to test his accuracy. Makes you appreciate quizzes more, right? At least you know where you stand.