It’s a widely accepted fact that Iraq had a chemical weapons program before the Persian Gulf War. Iraq and the UN cooperated to dismantle that program in the early 90’s. There was disagreement about whether that effort was exhaustive. The Bush administration’s claim to justify invasion was that there was an active ongoing chemical/biological/nuclear weapons program. Some old caches of chemical weapons materials discovered after the invasion doesn’t show that Iraq had an active chemical weapons program circa 2003.

You can read more about how the intelligence community failed here: https://en.wikipedia.org/wiki/Senate_Report_on_Pre-war_Intel...

For example, they thought that Iraq had restarted its nuclear weapons program, and there was nothing like that in the evidence you linked to.