thats not completely true. there is probably more nicotine in one bowl of shisha (1 cigarette: 0.75g, 1 bowl of shisha: 5-20g => more tobacco - more nicotine)
But the smoke of a hookah is completely different. you inhale the nicotine and exhale a big part of it again. when you smoke a cigarette, the smoke affects the lungs immediately and the nicotine stays in your body. thats why there is less nicotine in the smoke of a hookah.
but nicotine is not the biggest issue with smoking (sure, it is poison and it can make you addicted), but the most harmful part of smoke is the tar. tar is responsible for lung cancer and results from burning organic materials, what happens with the tobacco of a cigarette, but hookah tobacco (normally) doesnt burn, so there should be no tar in the smoke!
thats the difference in the harmfulness of different smoking