Except for the fact for in order for fluid to get inside the casing, it had to travel through the cement before it reaches either the shoe or the seal assembly. The positive casing test tests the seal assembly and checks the casing hasn’t burst - in BPs case (p83), it was definitely a good test. The negative test tests the shoe. Those results (p88) were very very squirrly.
Both the cement and the shoe must have failed for the blow out to have happened.
We can be pretty sure that there wasn’t any over displacement, as the plugged was bumped.
We can also infer that there was some sort of downhole problem prior to the cement job as due to the fact that it took nine attempts to establish circulation, and that after circulation was established, pressure was more than 200psi below what was expected. To me this points toward (though doesn’t prove) damaging the ball valve in the float, or damaging the shoe itself.