This is rather an attack on a strawman. No one thinks that ideas don't matter at all. Obviously you have to have a good one at some point.
When people say that execution matters more than the idea in a startup, what they're really saying is something fairly complicated: that you need both a good idea and good execution, but that (1) since good execution is rarer, you should worry more about that, and (2) good execution will tend to fix bad ideas, because if you pay attention to users and iterate fast, you will in the worst case end up with a good idea by successive approximations.
When people say that execution matters more than the idea in a startup, what they're really saying is something fairly complicated: that you need both a good idea and good execution, but that (1) since good execution is rarer, you should worry more about that, and (2) good execution will tend to fix bad ideas, because if you pay attention to users and iterate fast, you will in the worst case end up with a good idea by successive approximations.