Because NIST have confirmed the damage was superficial, i.e. had no bearing on the collapse which was induced entirely by plain old office fire… officially.
Why are you claiming the WTC1 debris damage, which affected other buildings too, had anything to do with it?
Perhaps he read it from wikipedia, as I just did:
I added the WTC 1 comment.
If you look up the  link, you'll see that it's supposed to link to a NIST appendix. I imagine they've changed the address of that file, however, as it doesn't currently link to a pdf file.  doesn't currently link to the indicated NIST pdf file either.