When Ronald de Sousa goes on to claim that poetry is “about language” in the same way that painting is “about paint” it is pertinent to question both parts of the statement.
I suppose this is meant as a gloss on McLuhan's "the medium is the message," though McLuhan was only pointing out that the means of communication necessarily shape what is communicated. A landscape is one thing, a sonnet another. From that, however, it does not follow that the message is about the medium. Poussin's Et in Arcadio ego is not about pigment. Wordsworth's "Tintern Abbey" is not about ink and typefaces. If the medium were the message in that sense there would be no message.
Philosophizing about poetry may be a dubious undertaking, but using poetry to express philosophy, as Lucretius and Thales did, may well be something worth looking into.