Just the idea that film music has the same place in the concert hall as the best music in the canon is a mistaken notion, I think
I actually completely agree with Williams completely here. I've never liked concerts that feature music outside of its intended context. I don't like when a single movement of a symphony is performed absent the whole symphony. I don't like a mish-mash of arias from different operas. And I don't care for film music concerts.
But that does not, in my opinion, make film music an inferior art form. It just makes it different. A single track from the soundtrack of a film score is not great art, but a film, in its totality, can be great art, and the contribution a soundtrack plays to that art cannot be overstated. It can uplift the film from the merely great to the sublime. And no living composer even comes close to the quality of John Williams.