Well, that’s one area where some people feel the Widom-Larsen hypothesis falls down: the neutrons are supposed to be formed from a proton and an electron. I’m not sure how that’s supposed to happen, but I can’t say that I’ve studied the hypothesis carefully (or intend to at this point).
I’m not sure who you mean by “they”. There are numerous actors in the LENR/cold fusion field, and they don’t always agree among themselves. There are a number of hypotheses that have been proposed to explain the phenomena that Fleischmann & Pons and other researchers have claimed to observe, and “D+D->4He” is just one—the first one, really. If you were reacting to Zawodny’s slide presentation, I can tell you that, although it may not be perfectly clear from the context, slide #10 is is actually dismissing the validity of the “D+D->4He” hypothesis. Like I said, Zawodny is a proponent of the Widom-Larsen hypothesis (slides #12 and #13).