Given the scientific evidence behind lucid dreaming, I wouldn't call it a 'claim'. If someone can, while in the midst of REM sleep, as determined by a polysomnograph, deliberately transmit a previously-agreed-upon signal to an external observer, that's reasonable evidence that the person in question is aware that they are, in fact, in a dream state.

Of course, if you disagree, it would be appropriate to research the phenomenon yourself.