A "7-8 inch" length of compressor spool is going to have lots of energy and is unlikely to be stopped by a lightweight nacelle cowling (the compressor case is designed to contain blades, but not large parts of the compressor spool).
Pure speculation on my part, but I'd bet money that one of those spool pieces sliced open the fuel line feeding the main engine fuel pump. That line is a couple inches in diameter, and the center wing tank boost pumps would be pushing fuel out the resultant opening at ~70 psi. Between the several seconds necessary for the crew to recognize the problem and command the shutdown, plus another couple seconds for the spar valve to actually close, that has the potential to be a whole lotta fuel.
While the penetration of a wing tank is certainly possible, I tend to agree with other posters than had there been a major penetration of a wing tank, things would have been even worse.