Not necessarily true.
> Showing bad ideas to be bad is very important in civil discourse, not being able to point out bad ideas by "retweeting" them with commentary will likely make this social network an echo chamber devoid of intellectual curiosity
Theoretically yes, but the problem with Twitter is not "properly" refuting ideas, because there is no such thing as absolute truth when it comes to human interactions. People will tend to believe whatever they want, independent of facts. Furthermore, people may interpret the same fact in completely diverging ways, depending on their prior conceptions.
Not allowing awfulness to spread could be more efficient than helplessly trying to refute it.