The principle(s) behind the claim is that the four seam fastball gives the ball the truest line of flight with all seams working equally through the atmosphere. ( in theory anyway)
The two seam fastball has a greater tendency to move, or as we like to say - has movement. This movement deflects, or moves, the ball from the straightest line of flight like its four seam cousin. Thus, the ole saying, just tweaked a bit - the straightest line between two points, or put another way, the shortest distance between two points is a straight line.... thus the fastest.
So, with the two pitches released with the same effort, the four seam has the greatest tendency to cover a straight line in the quickest time, Vs the two seam with moves slightly off that straight line, hence taking more time to reach its target 60 feet away.