Would it be a lack of abstract or real words that shows the modernist tendency toward using concrete language I have eaten the plums that were in the icebox from This Is Just to Say by Williams?

The poem is "This Is Just to Say" by William Carlos Williams. The poem is an apparent apology. The noun 'apology' is an abstract noun and an apology is an abstract idea, the Word and the act are concepts.

The author is using a concrete explanation (the physical absence of plums, the physical act of eating them, the physical properties of the plums as he ate them) to express the abstract idea (apology). The poem lacks abstract Words.

Note: There is one abstract noun in the poem:

...saving for breakfast...

The noun 'breakfast' is an abstract noun as a Word for 'breaking one's fast' after abstinence throughout the night. The noun 'breakfast' is not the food itself, it is the occasion that the food is eaten, a concept.