Not necessarily true. An IR seeking missile will only see the average of the heat sources and aim for that average. The Center Wing Tank is at the center of the infra-red averaged image of the four engines and the heat from the airconditioning units. At the velocity of the MANPAD (Mach 2.5) by the time it got close enough to distinguish any discrete heat source, it is too close to make any turns... It actually could hit anywhere on the plane... or miss entirely.
...It would have run out of fuel 3000 feet ago, and is coasting.
You may actually have something there, but not much. IR seekers go for the heat, and with no countermeasures happening, it will lock on to an engine, and go there. There have been a couple of MANPAD attacks on Commercial jets in Afghanastan, and both hit the engine, not the fuselage, and those jets were well within range of the missile.