The problem with this scenario, as presented, is that it assumes that "sabotage" is a binary variable. If that were the case, the pool of possibilities would consist of: (1) Fifth Column exists & sabotage occurs, (2) Fifth Column exists & sabotage does not occur, and (3) Fifth Column does not exist & sabotage does not occur (presuming that sabotage, as defined in the scenario, could only be accomplished by Fifth Column). In that case, necessarily, lack of sabotage could only reduce the probability of (1), and therefore could only reduce the probability of the existence of Fifth Column.
However, sabotage is not a binary variable. Presumably, there is some intermediate level of sabotage (call it "amateur sabotage") that one might expect to see in the absence of a well-organized Fifth Column. In this case, we need to add 2 possibilities to the pool: (4) Fifth Column exists & amateur sabotage occurs and (5) Fifth Column does not exist & amateur sabotage occurs.
Given the above, the complete absence of sabotage would reduce the probability not only of (1), but also of (4) and (5). Depending on the prior probability of (5) occurring relative to the probabilities of (1) and (4), Warren's argument may be perfectly reasonable.