You would need to go back to the original thread to find that assertion, if it’s still there and hasn’t been deleted.
Addendum: That thread is “What are you reading?” and on it, near the bottom, Benkyo said, first, “every work of art makes some form of argument,” and second, in a later comment, “For example, an argument made by David is ‘this is a beautiful man’.” (I’m not sure that he’s talking about Bernini’s David, which is what I was referring to; his comment seems more likely to apply to Michelangelo’s, or Donatello’s.)
Even accepting the use of “argument” to mean “assertion,” I don’t think that it makes any sense to claim that about a work of instrumental music. An assertion must be an assertion that X, and music doesn’t assert any propositions at all.
As for Bernini’s David (which you can see at Wikipedia), I will grant that it has a theme, which is the tension of the figure and its focus on the target (it shows David before he kills Goliath, not after). But I don’t think it asserts anything. You could take it as “this is a beautiful man” (though the expression of the face is not what is conventionally thought of as beautiful), but you could take it as “this is a man chosen by God to fight his battles” or “this is defiance of power” or “this is an arrogant and aggressive jerk”; different people might get any of those from it.