Communication is not the recitation of mere facts: "Picasso said 'after Altamira, all is decadence.' The ratio of the diameter and circumference of a circle is pi.".

Nor are we like the Tamarians of Star Trek, where mere cultural references have deeply shared denotative meaning. "Picasso, commenting on decadence after Altamira. Shaka, when the walls fell.".

We use other words around these things we say to express the thoughts in our head and try to replicate them in someone else's.

For what it's worth, here's the actual paragraph from A History of Ancient Britain in whose context this sentence was originally written:

The Upper Palaeolithic cave art of Europe was a tradition that lasted for perhaps 20,000 years and it will always be rightly described as primitive. But it is upon those anonymous artists' shoulders - giants' shoulders - that later masters like Picasso were able to stand. The mercurial Spaniard himself declared: 'After Altamira, all is decadence'.

I mean, sure, this is probably not a great use of the word 'mercurial'. Maybe the author thought that Picasso's volatile moods mean the fact that he said something so profound on such a subject lends the quote some particular poignancy? Personally, the idea that a person of a mercurial temperament might dismiss the entire arc of Western art as pointlessly indulgent seems totally in character. I suspect Byron probably said all of literature after Aristotle was repetition during one of his moods, too. So probably not the author's finest turn of phrase, for sure.