Seems to me that given the forecast and reported (by the Captain to his Company) "hard" turbulence due to dynamic CB activity over the area they were in, and the aircraft was operating at 35,000' loaded with fuel for Paris, the starting point for speculation (if you must) would be the possibility of a jet upset/loss of aerodynamic contol situation if the aircraft entered moderate-to-severe turbulence (such as you'll find above and around developing CBs), warmer ISA deviations than forcecast with even light to moderate turbulence, or an unfortunate combination of both.
Heavy weights while operating from the mid-30s up combined severe turbulence can lead to a hairy "test-pilot" situation very quickly. Cruise altitude tables etc. do not account for it, and although an airliner is stressed to survive severe turbulence encounters with "merely" broken equipment,components and/or injured passengers and crew while the wings stay on, the stresses involved in recovering from a jet upset/loss of aerodynamic control at altitude can quickly exceed those parameters. Having to recover from one at night while possibly descending through or into the CBs that put you there in the first place reduces everything to a strong odds-against situation.
Data messages indicating failed/failed electrics and/or components during prolonged moderate - severe turbulence as they get bounced around wouldn't suprise me, but the sudden loss of an aircraft directly due to a CB-generated lightning strike is almost a non-starter theory given what you're also finding along with it (possible extreme turbulence, hail shafts) while operating heavy at high altitude.
Just my 2 cents, and a tangental reminder for all of us operators to heed the red flag of moderate or > turbulence being forecast at altitude enroute and carefully consider the aerodynamics for our given weights and temps for likely encounters. You never want to get into a boxed-in situation where conditions dictate you must descend to maintain aerodynamic control but descending also means you're no longer able to deviate around tops of CBs but rather find yourself more in the #@$@.
Oh, and anyone downplaying CBs because "they flew through one and it came out Ok" is, quite simply, an idiot for so many reasons and at so many levels they don't belong in a cockpit where they have to routinely deal with grown-up weather conditions. They sound like the worst kind of severe-weather neophyte probably not even understanding the the differences between airmass and steady-state and what they produce, what K Index and Skew-T are, and couldn't you the relationship between the reflectivity dB on their own contouring airborne radar and it's relationship to the probability of turbulence/severity and hail/diameter encounters let alone the basic, proper use of tilt and gain.