Hmm...yeah that explanation for the origin of the X-Wing makes no sense to me. In the opening crawl it's explained that the theft of the Death Star plans is the Rebellion's first victory against the Empire.
And yet their fighters are all worn and show signs of age and heavy use. In my mind, all of the Rebel Alliance's equipment was scrounged second hand. The fighters dating back to the days of the Republic.
They are superior to the Empire's fighters but that's because the Empire decided to cut costs as there hadn't been any real opposition up to that point.
That leaves the Rebellion with superior equipment but facing overwhelming odds by the sheer size of the Imperial fleet.