The 9/11 Commission stated part of the problem was a "failure of imagination." Clarke's 1998 scenario blows that away (no pun intended). He clearly imagined a similar attack, but nothing was done to prevent something so plausible. The true failure of 9/11 was beauracratic inaction and obstruction (see the Gorelick Wall).
Getting back to your original point, I sincerely doubt the MSM would dust off Clarke's memo where these chemical attacks are concerned. They're only interested in finding the portions critical of the current administration. Talk about cherrypicking intelligence!