The criteria was merely influence and nothing more. He made the point that it's not a list of literary masterworks (or else Dickens and Goethe would be there)nor is it a list of the best known books of all time (or else 'Gone with the Wind' would be there) nor is it a list of the books he would ahve wanted to have influenced people (or else Winesburg,Ohio and the work of Lewis Carroll...)
The Influence will be apparent. It is as, or more, significant than the inclusion of Orwell. I would further suggest, judging by the contemporary inclusions to his list, that it is the content Seymor does not agree with rather than his tired, tawdry, attempts to dismiss the work via its prose.