From my observations over the years, there can be a tendency to underestimate what actually constitutes severe turbulence. For the record, you don't have to be out of control to be in defined severe turbulence. The definition is "may", but needs to be taken in the context of what else is going on at the time;
"Conditions in which abrupt changes in aircraft attitude and/or altitude occur; aircraft may be out of control for short periods. Usually, large variations in air speed. Changes in accelerometer readings greater than 1.0G at the aircraft's centre of gravity. Occupants are forced violently against their seatbelts. Loose objects are tossed about".
I recently experienced what I deemed to be severe turbulence during a go-around in foreign airspace. Naturally there was no accelerometer to give me instantaneous G-readings but I did observe speed fluctuations of around 40 knots (20 either side of my target Vref + 20) and although the autopilot managed to hold on, I was certain that it was not commanding rates of climb varying rapidly between 500 and 7000 fpm!
In the heat of the moment, the only real test I had at hand was the "eye-roll" test - the instruments were a blur and I can't even describe the noise. Test failed! This was further backed up by data some days later which confirmed a total fluctuation range of 1.1G and 0.8G over 3 seconds on 3 separate occasions. Truly frightening, but Boeing (and Airbus) do build 'em like tanks...
All up sounds like the EY crew did a great job (hope they found a way back to PER which didn't force them back through the bumps!). We should never be afraid to call severe turbulence what it is, even if uncertain that it's bad enough to cause a fuss. If doubt exists? It's probably severe!
ps - I also note that one Australian carrier does tend to call severe turbulence when everyone else assesses light to moderate. Overabundance of caution, perhaps?