The prof is clearly right. But like “I could care less” instead of “I could not care less” it’s a lazy illogicality that we let slip by. In the case of “X times as many” used in a negative sense it definitively classifies the writer as a journalism school graduate.
I, too, agree with the English professor. Although you can convince yourself that you understand the meaning, the usage is sloppy at best.It does not help the reader as much as it should and, like writing in the passive voice, it gives the impression that the writer himself may not know what he is talking about.