For one, there was no first hand evidence of other limestone plugs above. I believe I recall that was a presumption for the rough surfaces and debris in the tunnel. Also, the story of the prism stone is very suspect considering the men who were digging the tunnel supposedly heard something drop after blasting, and it was reported they thought it was from that general direction. But this account was supposed to have happened when they were still several feet of stone away from even breaching the DP. So it sounded very unreliable and 'creative' I suspect.
As for these two details, my suspicion of plug removal throughout time I had presumed hypothetically, would do quite a bit of damage to the lower section of the AP and upon each removal, I presumed there might be some patch work done. It goes along with that presumption that these patches might not adhere permanently and might detach and collect at the bottom. Its not possible for them to have had this perspective at the time of discovery, so its reasonable to comprehend their suggestion of the reason, but the final take away was just that. A suggestion.
The prism stone story was even less linked to actual observation, so I thought it more possible, considering this new exploration into the real purpose of the pyramid, that what ever they heard could just as easily have been a special stone assembly specifically meant for plug jacking that could have become dislodged, fallen down the DP and broken up. There was no evidence of stone remnant of either this possible mechanism or the Prism stone so its all conjecture at this point.
I wonder if the tunnel ironically leading to the exact place where the plugs ends is possibly as a result of original robbers, (the ones who first saw the AP in a virgin state), might have originally scrambled up the tunnel leading to the grotto and queens chamber, scrambled down the AP and made rough measurements as to where to lead their excavation tunnel, more than likely for the purpose of removal of items in the pyramid. Because if this were a possible explanation for this coincidental tunnel connection, it confirms the vacant AP. Just a thought.