It is a big statement. I've personally not used an alarm clock in about 10 years, I wake up naturally. The times I do use an alarm clock I can genuinely say often make be feel ill for the rest of the day.
Given the critical importance of sleep (if you don't have enough, you die or go insane) - the default assumption should really be that anything that interrupts natural sleep/wake cycles is damaging, and we should have to prove otherwise. I mean, why not? You will notice the effects of going without it much sooner than going without food, for example.
Personally, I totally buy that it could be as bad as (or even worse) than smoking. How much bad stuff has happened due to lack of sleep/quality of sleep that we just discount as due to something else entirely?
Still, not sure it's as bad as smoking.