You're correct in that being in the "coffin corner" is not dangerous in itself. In straight-and-level flight, if you go any faster, the extra drag will slow you down. If you reduce your thrust from your engines, you will slow down and lose altitude.
The problem comes about if you enter into any unusual attitudes, which is certainly possible in severe turbulence. The most important part would be a safe recovery to straight and level flight. An unusual attitude can put you in a situation where you overspeed the aircraft (e.g. nosedive). Above the maneuvering speed of the aircraft, you must be very careful to regain control without overstressing the airframe.[1] If your speed sensor(s) are giving you or the autopilot faulty readings, it would be very difficult to know the safest way to regain control of the aircraft after being put into an unusual attitude due to turbulence.
[1] American Airlines Flight 587 crash (NY, 2001) taught us that even below the manuevering speed of the aircraft, the airframe can be overstressed due to dynamic loads from pilot control inputs.
The problem comes about if you enter into any unusual attitudes, which is certainly possible in severe turbulence. The most important part would be a safe recovery to straight and level flight. An unusual attitude can put you in a situation where you overspeed the aircraft (e.g. nosedive). Above the maneuvering speed of the aircraft, you must be very careful to regain control without overstressing the airframe.[1] If your speed sensor(s) are giving you or the autopilot faulty readings, it would be very difficult to know the safest way to regain control of the aircraft after being put into an unusual attitude due to turbulence.
[1] American Airlines Flight 587 crash (NY, 2001) taught us that even below the manuevering speed of the aircraft, the airframe can be overstressed due to dynamic loads from pilot control inputs.