The only thing that's ridiculous is a simplistic sweeping statement like that. For one thing, no one even said "more work" necessarily, although that is absolutely perfectly possible and valid, but better work, and better overall performance as in morale, loyalty, effectiveness (better imagination and problem solving), longevity (less turnover, more return on training investment, better institutional knowledge retention), etc.
I don't think anyone is necessarily saying more work will get done although that's possible I guess, my understanding is that in a humanitarian sense cutting down expected working hours would increase employee satisfaction/happiness without affecting productivity much if at all and that's the main draw of the movement.