"easier to solve" is not the same as "prevent". All open source licenses make fragmentation easier to solve.
Whether some licenses are better than others at solving fragmentation is a complex question. How many projects have been started simply to avoid the more onerous restrictions of the GPL? LLVM, Android, etc... would all this fragmentation had happened if these projects were under a more liberal license? Seems unlikely.
So, in the real world, I don't think that it can just be taken for granted that the GPL is the best license for avoiding fragmentation. It's a seriously complex question.