What is meant by the quotation "what to him / Are Plato and the swing of Pleiades?" in "The Man with a Hoe" by Edwin Markham?

In this complex stanza, points are being made in reference to a Biblical allusion to Psalm 8:5: "You have made [humans] a little lower than the Angels and crowned [humans] with glory and honor" (Psalm 8:5). To describe laborers—as seen by him in the oil painting "L'homme à la houe" (1863) by Jean-François Millet—Edwin Markham enlarges the Biblical concept of "a little lower," making it a great "gulf" of separation between the laborer and the angels (seraphim). He assigns "Time's tragedy" to the laborer's "aching stoop." Dramatizing his point for the learned (non-laborers) who are reading his poem, he suggests that such stooped, lowly, "profaned and disinherited" laborers have no strength to consider the problems Plato poses or the theories he presents; they have no opportunity to unbend and turn their eyes skyward to contemplate the meaning of the movement of the starry Pleiades. 

What gulfs between him and the seraphim!
Slave of the wheel of labor, what to him
Are Plato and the swing of the Pleiades?

Markham goes on to suggest in the lines following "Pleiades" that the "dread shape" of the laborer, through whom "the suffering ages look," does not have the time, strength, or ability to contemplate the mundane, earthly pleasures of the "peaks of song, / The rift of dawn, the reddening of the rose." Markham makes the point that grueling labor reduces humans to being far lower than their God-given state, reduces them to a state that is deadened to higher thought and contemplation, to a state that is as stooped as their physical state:  

Time's tragedy is in that aching stoop;
Through this dread shape humanity betrayed,
Plundered, profaned and disinherited.

The larger point that these lines are part of, "what to him / Are Plato and the swing of the Pleiades?" is that "this dread shape humanity betrayed"—plundered of rightful thought, profaned of rightful place, disinherited of the the wonders of life—cries out in protest to the "Powers" that created life. His cries, which are silenced along with his contemplation of Plato and his wonder at how the summer and winter "swing" of the rising and setting of the Pleiades cluster, are a prophecy of retribution for those humans who lower other humans in this way: "Cries protest to the Powers that made the world, / A protest that is also prophecy."