a page-long prose poem, “The Dying Castles,” which appeared in a 1968 issue of the British SF magazine New Worlds (#200)
The article goes on to make the intriguing claim that Delany has no memory of having written any part of it, and that having his name attached to it "must be a jape".
I have not found any statements by Delany concerning the article - most sources seem to be verbatim quotes of the Wikipedia article. Have Delany, Moorcock, or Sallis, the authors to whom the poem is attributed, made any definitive statement about the authorship? Unfortunately the Wikipedia article does not cite its source (and I would be interested to read it!). As a side note, the poem was not published in 1968 as the article claims, but in 1970 which casts some doubt on the article's general level of accuracy.
The text of the poem is available on the Internet Archive.