I honestly can't say that I've heard this.
What I have heard, and completely agree with is that the methodology of measuring the heat generated is dubious. As an engineer with a little experience in setting up and monitoring equipment - this perplexes me. It's just not that hard, nor that expensive. In fact, many equipment Reps I know would FREELY donate equipment on loan; for these measurements to be precisely taken at numerous (250+) points, both gas as well as surface temperatures. So, while I am fascinated by the prospect, I am also disappointed by the lack of thought put into the demonstrations. However, I'm ever hopefull that someone will press forward and present a solution to our energy problems.
Frankly, I don't give a hill of beans if some scientist doesn't understand "Why" it works. I don't understand "Why" women do what they do, but I'm certainly grateful that they do what they do.
But, I've never heard a scientist say that is't patently impossible.
Too bad you agree, because the "dubiousness" exists only in the eyes of the knee-jerk antis, and arises almost entirely from their FUD suppositions. If you read the actal reports generated by those who actually did the tests, a different picture presents itself. Those reports can be found at the LENR/CANR website. I suggest reading those, and watching the accompanying videos.