Hardening with any plant sprouted indoors is required before they are put under full sunlight. It's not unique to cannabis.
(Your stems could have also used some further depression into the grow media and some piling up of earth around them (always below the leaves) to support them, too. Probably too late for that now though. Next time.)
Growing conditions, degree of southern exposure, and shade conditions are different for everybody. It depends on where their house is, their neighbors are, trees growing around them, etc. These things can affect the individual outcome dramatically. So you are going to hear a cross-section of opinions based upon experience as to what is "best" to do.
My point: what is "best" varies a lot. It is dependent on local geography and effects which are highly particular to that unique location.
I don't know about hardening for 7 days. That seems excessive to me. But 2 days normally are required as a baseline that everybody agrees upon. Some more conservative growers might be growing in circumstances where their geography tends to demand an easing in to the sun over 3 or 4 days.
Whatever the case, every plant needs a little bit of time to acclimatize. Your plants are not dead. They should recover.