I would think that your story of the f/e hitting the ceiling would qualify as one of those severe encounters.
It was without question an severe to extreme encounter, borne out by control issues experienced in the cockpit. It was a very violent ride.
I don't have a specific number, but would estimate several hundred severe turbulence encounters over the years (nearly all at low level in strong winds in mountainous terrain), with several dozen others in the extreme catagory.
As you stated, the aircraft may be momentarily out of control. This however, does not mean that it is and therefore my wonderment why there would be radio calls seemingly doubting someone elses call.
The problem, as stated, is that a turbulence call is subjective. I wouldn't second guess another's call, but at the same time I've known a fair number of crewmembers call "moderate" to what I'd have called "light" at best...it would seem that for some a ripple in the coffee is turbulent, and when the rolls or bumps are just severe enough to cause one to gently nod off to sleep, some seem to be nearly screaming "get me out of here!"
That said, I think it would be a grave error (or certainly could be) to presuppose what another crew might be thinking. An airplane on a given track may experience significant turbulent action, yet the preceding and following aircraft experience nothing. It happens. Wave, wake, orographic, jet-stream induced CAT, etc...not everybody finds the same bumps in the road for a multitude of reasons. To discount what the other crew has reported would be a mistake.
On a leg last year between Honolulu and Los Angeles, we experienced what I'd classify as "ugly moderate" to severe. If it wasn't severe, it was close. It didn't last long, but while we did report it, aircraft ahead of us and behind didn't report it, and aircraft on parallel tracks didn't experience it, either. I believe we reported it as moderate to severe. I think the report was accurate enough to convey what we experienced.