Yes, someone did claim that video footage was 10 years old. CNN disputed the claim. Other news agencies also had similar footage, and a day or so after the attacks, the Palestinean authorities forbade news agencies from covering the celebrations. Furthermore, the Palestinean ambassador was interviewed about the celebrations on Sept 12th and didn't deny they'd happened. She correctly pointed out that those celebrating were only a tiny number of people out of a population of millions. All of this strongly suggests to me that the celebrations, small thought they may've been, did indeed happen.