More discussion is always welcome, but as covered by /u/bigbluepanda and /u/ParallelPain here, bushidō was, in many ways, a modern invented tradition by Japanese intellectuals attempting to present Japan to the West by picking and choosing older elements to craft the kind of narrative desired. Later on, it was codified, again, by the Meiji era government as an extension of a largely romanticised past. Nitobe's views on bushidō are in particularly noteworthy as being unpopular in Japan, with his book aimed at Western audiences first and foremost.

To answer your question more directly: it's difficult to say that bushidō was as important as Nitobe claims. Not only is the author far from reliable, but prior to the widespread publication of works like his own and others, there simply was no universally recognised "code of bushidō".