Missiles don't explode and "become" shrapnel. They contain a warhead containing shrapnel. A warhead designed to explode in a very specific way.
And when that shrapnel hits near the front a commercial airliner with a pressurised cabin at FL330, travelling at M0.82, it generates a whole heap more shrapnel that used to be part of the aircraft, which the rest of the airframe will plow into at high speed. As the OVV report explains in some detail, the missile did enough damage to separate the entire cockpit and front portion of the aircraft from the rest of the fuselage; How you could break up an internally pressurised airframe in this way without generating further shrapnel, to strike the wings, engines, and other parts aft of the initial impact is a total mystery.
Catastrophic failure of a pressurised cabin is quite easy to achieve; as the Lockerbie bombing of Pan Am 103 showed, even a small bomb that can be hidden in a tape recorder can destroy a large jetliner, due to the effects of internal pressurisation combined with a slipstream at nearly 1,000km/h. A small hole doesn't stay small for long in such conditions.