It was very much synergy. Peter Pan's Flight, Snow White's Adventures, Mr. Toad's Wild Ride, etc were all based around movies, with the intention of the ride promoting the movie and the movie promoting the ride. Just because some were "flops" does not mean no synergy was intended. Also, Davy Crockett's Explorer Canoes were cross-promoted with multiple, very successful TV specials on Davy Crockett. Synergy was high and in all forms.
Perhaps the biggest example is the park icon and centerpiece, Sleeping Beauty Castle. It opened with the park four years before that movie came out. In fact, the castle was supposed to originally be Snow White's, until Walt decided that he would make the movie Sleeping Beauty, and so he made a last minute change to promote his future movie. That is true, undeniable synergy right there.
Walt was a big of a synergy guy too, he was just smart about it. And he realized that synergy can come from a variety of places, not just movies.