I do not think that Steinitz was saying that Morphy won just because his opponents made book blunders in the opening. He was disputing the assertion "that Morphy’s style was all genius and inspiration throughout, while the play of modern masters is all book and study". With regard to opening novelties, there is something of a problem because agreement depends on agreement on what is part of the opening. It might be of interest to look at the Dover reprint of Paul Morphy and the Evolution of Chess Theory by Shibut. Much of the Steinitz commentary is reproduced.
http://store.doverpublications.com/0486435741.html
Hello everyone. Steinitz had this to say regarding Morphy:
"But when it is so freely asserted that Morphy’s style was all genius and inspiration
throughout , while the play of modern masters is all book and
study, I would take leave to answer frankly that just the very reverse can be
proved in the only part of the game in which knowledge and study can be of
much use and in which a test of the assertion can be applied, namely in the openings. For
Morphy possessed the most profound book knowledge of any master of his time, and he
never in his practice introduced a single novelty, whereas since his day the books have had
to study the players."
Do you agree with this? Morphy never made novelties? So his opponents just made book blunders and Morphy used to win? Is that the idea that Steinitz is trying to prove?