You should never rapidly change anything in your tank, unless its nitrate removal or something like that, so I'd say just how they market the product, it seems like
a.) They're acknowledging that it isn't a universally acceptable thing to jump your calcium through the roof, so with the caveat to say that its for experienced people, if you get a precipiation event or crash, they can always say you didn't use it right and
b.) They're marketing to people who are generally less informed and want a quick fix for a hobby that takes a lot of work, paying attention, and careful maintanence.