Is this possible? My first thought was "infinite monkey syndrome." Given enough time, there eventually will be an octopus lair with bones that make a reasonable facsimile of Botticelli's The Birth of Venus. Perhaps, but not terribly likely, even with the sorts of time they are talking about.
So now I ask myself, how does it affect what we know now if this self-portrait assertion is true? Other than being a terribly interesting fact about a long dead animal, I can't really think of any way this changes our reality . We have already established that octopus are pretty smart without assessing their skills of an artist. The real question, for me, is whether or not I want to live in a world where horrific sea creatures in some dark past would devour other monstrosities and decorate their lairs with the remains.
Yes. Yes I do.