One thing's for sure, it would have meant a lot more damage in Metairie. And this McBride post indicates it sure could have happened that way. More to the point, though. Maybe it still could.
Given the shallow depth it was found in 2011 and 2014, it makes sense some of the debris was part of the I-wall and levee construction in the early 1990's. That work did involve placement of dirt on top of the existing levees at the time. It extended the entire length of the canal (all the yellow and red lines shown above). Other, deeper debris has likely been there far longer. The records of nearly every soil boring taken in 1981 along the west bank - long before the I-walls were put in - have numerous mentions of shells, wood, bricks, and other "miscellaneous fill." Those records were used in the basis of design for the Corps' early 1990's work. This junk has been known to the Corps for a very, very long time. It also very likely extends the entire length of the canal on at least the Jefferson Parish side.
How it got there doesn't really matter now though, because all the junk is still there today, and the integrity of the outfall canals remains the responsibility of the Corps. As the decision recorded in the contractor quality control log of March 11, 2011 indicates, there is no way the 17th Street canal west bank levees would be built today with the debris-riddled dirt that is currently sitting in them. That dirt is - according to the Corps - "unsuitable." Yet there the levees sit.
Correction: Originally this post cited a breach in the Orleans Avenue outfall canal. Orleans Avenue did not, in fact, breach. Here's that T-P Katrina animation in case anyone else needs a refresh.