It's not ironic. He made his living (little as it might have been) by doing what he wanted to do. He wasn't cleaning toilets on the side to supplement his hobby of composing. Other people making more money off his work doesn't change anything. His passion supported him financially.
He wasn't relying on copyright laws, which is what the parent poster was implicitly talking up with his statement; i.e. a world without copyright laws did in fact produce a Mozart, and it will keep producing it regardless of legislation.
