I think he was lumping the celebration world wide as thousands - not just from NJ.
lumping,..yes that was it.
his statement was :
“”I watched when the World Trade Center came tumbling down, and I watched in Jersey City, New Jersey, where thousands and thousands of people were cheering as that building was coming down,†he said.
http://www.businessinsider.com/donald-trump-new-jersey-cheering-9-11-2015-11
somehow I think he may simply have mispoken.. for some reason