For being about poetry, "Proem" is pretty loosey-goosey. It doesn't rhyme, it doesn't have a regular meter, and doesn't really follow any poetic structures. Maybe that's part of why some people consider it to be a prose poem, even though it also doesn't exactly fit into that tradition either, because it does have poem-like line breaks. Instead, all of the lines except for the last two end in semicolons, as though the poem were just one, long run-on sentence. And, really, it kind of is.
So what does this free-for-all do for the reader? It feels like an improvised creation, and since it isn't methodically structured, it seems a little more dream-like, which goes with a lot of the poem's surrealist imagery. Try the opening lines on for size:
At times poetry is the vertigo of bodies and the vertigo of speech and the vertigo of death;
the walk with eyes closed along the edge of the cliff, and the verbena in submarine gardens; (1-2)
All those "ands" in the first lines, together with the crazy undersea garden image and the lack of a regular rhyme or meter make the poem sound like the poet is making it up on the spot, as though he were in a trance and writing down whatever came to mind. Pretty trippy, right? The meter, or total lack thereof, then reinforces the wild, dreamy nature of the poem's feverish exploration of the possibilities of what poetry might be.