This should be moved to the Literature Discussion Forum so you can get viewpoints from more than one perspective.
I would argue that the "greater journey" undertaken by the Americans portrayed in the book doesn't have to do as much with geographical travel as it does with personal education and expansion. Yes, there was a great deal of learning gained by all the individuals featured as they encountered the different perspectives and points of view in France in general and Paris in particular.
However, the internalization of those new experiences led to personal revelations, new avenues of expression, new ideas and forms of thought. Those discoveries carried a number of the persons described in different directions and possibly farther than they would have gone without the time of learning and growth in Paris. I see the title as reflecting the journey toward development experienced by the Americans during their time in Paris.
So why the comparative-"greater?" Of course what you say is true, but I have a feeling McCullough meant more than that, and more even than his little comment "Not all pioneers went west."