I think you are right: the understanding of what art is, but much more even the way it functioned in society has changed a lot since then.

But even then noone would have told someone that he could not be an artist (whatever that meant then) because of themedium he or she used.

Think of it as of a pen: people use it to write shopping lists. People also use it to write literature.
It's not the medium. It's how, and to what end, you make use of it.

Photography is a form of art. Not all of photography is. But definitely some of it.