If Fredo was competent, then that could change the whole complexion of things. Even if Vito still was shot, Sonny would have had Fredo as one of his resources and who can say how that might have affected the way Sollozzo acted. Sonny being hot for his deal alerted Sollozzo because he considered both Fredo and Michael non-factors. He gets rid of Vito and he only has Sonny to deal with. Fredo being competent changes the landscape.