The problem with this argument is that art is a humanity, and therefore subject to the changes of the society which creates it. Saying that art means nothing because it can't change is like saying the names of countries mean nothing because their borders change.
What is art to someone 100 or 1000 years ago is not the same as what art is to someone 100 or 1000 years in the future just as what any country was to someone 100 or 1000 years ago is not the same as it will be to someone 100 or 1000 years in the future.
In essence, it's a theseus' ship problem, and by declaring art meaningless just because it does not have an objective independent definition is throwing the baby out with the bathwater. What is art ostensibly is what we as individuals and social collectives choose to define as art. Nothing more, nothing less, as all human constructs are.