I understood the point perfectly well. The point's wrong, is all. It's kissing cousin to the old saw, "Perception is reality."
Satan is not the hero of Paradise Lost, regardless of whether later generations had a desire to see him as the hero. The poem simply was not constructed with him as the hero, even if Milton's ambiguity about rebels made him give the devil his due. The poem ends with theodicy, not Satan's tragedy. Milton's theodicy may no longer carry conviction, leaving the poem more honored than read, but that's not the same thing as being a poem with Satan as the hero.
But suppose by a freak of nature, the meaning of Paradise Lost was imbued by the reader. It's a long poem, twelve books if I recall correctly. Surely, each book was imbued with meaning during the long reading process, no? And each scene in each book, too? And for that matter, each line, right? Of course, each line was composed word by word, and in the process of reading each line has the added meaning of each word. Except everyone knows that words have their meanings given, not ascribed.
A work of art operates like new words, or the evolution of new meanings for old words: The user gives meaning by context. (Occasionally by formal definition.) The hearer/reader determines the meaning by interpreting the context. Loss of artistic control or dishonesty can leave the context contradicting the manifest meanings, but these are flaws in the artistry. Refusal to generalize from context, or selection from context is misinterpretation.
In religious art or monumental art in strongly hierarchical societies, the context is highly restrictive. In those cases, the notion that the reader/hearer/viewer is ascribing the meaning is patently absurd. The insistence that the artistic intent is irrelevant ignores the very existence of much of humanity's artistic work! I suggest there is a prescriptive intent, not a description of reality. The question is, what qualifies the proponents to prescribe?
That reminds me. Although no one gored the "art for art's sake" ox, we heard it bellowing anyhow. In the end, another aspect of art is simple decoration. That is the category "art for art's sake" falls into. Given that there is really no disputing taste (as opposed to meaning!) an announced taste or distaste for decoration/"art for art's sake" permits of no discussion. There is only the tacit presumption that same have good taste and others have bad taste. I suggest that in the end, this presumption reflects commitment to one's social milieu.
