Wrong. On several counts.
The area around WTC was fill material. Go find the article I linked to in the "Bridge". That shows what the shoreline of the Hudson River looked like since the original settlers. The river has been pushed back several times, and in fact was pushed back by using the excavation material from the WTC to reclaim additional land.
What you call the slurry wall is generally called the bathtub. A trench was dug in the ground, and slurry pumped in to maintain the shape of the trench. This slurry then had rebar cages inserted in the trench. After that, concrete was pumped in, which replaced the slurry. Bingo, a poured reinforced concrete wall that used the ground as the forms.
The bathtub wall was damaged in several spots, but NOT destroyed. Much concern was had about reinforcing the wall while the recovery work was ongoing. If the wall had failed, the site would have been flooded. No additional wall was built, to my knowledge. The existing wall may have been reinforced, but I'm not aware of that.
And Manhattan is solid bedrock, once you go down far enough.
Some supporting articles
http://travel2.nytimes.com/fodors/top/features/travel/destinations/unitedstates/newyork/newyorkcity/fdrs_feat_111_9.html?n=Top%2FFeatures%2FTravel%2FDestinations%2FUnited+States%2FNew+York%2FNew+York+Cityhttp://www.manhattanusersguide.com/archives_content.php?contentID=020703&category=infohttp://www.geotimes.org/nov01/NNwtc.htmlLink to this site and find the PDF that includes pictures:
http://www.nae.edu/nae/bridgecom.nsf/weblinks/CGOZ-58NLJ9?OpenDocument