Severe and extreme turbulence does exist, have no doubt. Been there, done it, didn't enjoy the roller coaster ride one bit, but never doubted for a moment that we would not survive this. (To put it into perspective, some poor souls have not survived the experience).
I've enjoyed the atmospheric 'super' roller coaster 3 times in 28,000 hours of flying, about once every 10,000 hours, probably par for the course.
I've survived numerous other "very rough rides", but I would not call any of them severe or extreme, just unpleasant. Several times I've heard reports of SEVERE turbulence ahead of us from other aircraft on the same track, battened down the hatches in advance, and wondered afterwards if we'd been sharing the same sky as the other aircraft.
It is my personal opinion that most pilots over-state turbulent conditions, but then again, that's just my opinion.
Regards,
Old Smokey