In order to best answer your question, I need to first address the phrase, "truly about," and its relation to analysis of poetry. It can rarely be said that a poem is "truly about" anything, unless of course we could sit down with the poet and ask them face-to-face. As a poet myself, I do often have a specific idea when I compose a poem, but that idea can often change during the writing process. I have also had readers see something in my poem that I never intended, but that helped them understand the poem, and be moved by it, in a unique way that I appreciated and respected. This can be said of most poets, and I'm sure William Carlos Williams would be fascinated by the diversity of interpretations "This Is Just To Say" has received.
All that said, having read most of Williams's work, and having frequently heard his life and work discussed (especially this poem), my guess as to the "true meaning" of this poem, is that it really is as simple as it seems. It isn't necessarily just about the plums, but about the person eating those plums and writing that note, and about the person the note is directed toward. So much is said is so little a space. Not only does the short nature of the poem and the word choices the speaker uses remind us of a sticky note left on a fridge, but it speaks to the nature of the relationship between the writer and recipient.
The title (which also serves as the first line of the poem) "this is just to say," reveals a certain level of intimacy and familiarity, a sort of "I just wanted to let you know" or even a kind of "hey, FYI" like we might send in a text. You can picture the note left on the table, the icebox next to the stove, the empty bowl the plums might have been in, just through the everyday, familiar tone the speaker uses. The second stanza, in acknowledging that the plums might have had a specific purpose, shows the speakers sympathy for, and respect of, the other person's needs. But the next line, even though it begins as an apology, gets a little snarky, a little "sorry, not sorry," and shows another side of that familiar relationship, the familiarity of humor.
It really is a very simple poem, and in its simplicity, a charming, and rather funny, snapshot of everyday life. This, like "The Red Wheelbarrow," is the kind of poem that made Williams famous, and through his work, changed the direction of modern poetry. Williams's work shifted much of poetic tradition from complex, multilayered poems, to simpler, more minimalist, but still effortlessly elegant, celebrations of everyday life. And that, I believe, is the simplest and truest description of this poem; it is a bright, bubbly celebration of human life, of its humor and honesty, of the simple joys of eating a ripe plum, especially one that someone you love was saving for themselves.
I hope this answer is helpful! I would love to discuss this and any other poem with you, and help you discover even more about how poetry works.