Powell knew that some of the stuff in his speech was based on flimsy evidence and threw out even more material that was feebly sourced, despite Cheney's people (who were behind this) opposition (this is all well-sourced by witnesses who were involved in preparing and fact-checking the speech). He was a moderating force in the administration and he didn't want to play fast and loose with facts, as he ultimately did.
He was determined to do his duty to the President because the alternative was to resign, and soldiers don't do that. Once on board with the White House campaign to sell the war, he got (consciously) compromised, not only in his UN speech, but in selling the war to the American people.
If I had to guess how much he really knew, let's say he hoped some of it was true, because no one really knew what was untrue, as the US intel on Iraq was skimpy.
This is far from what types like Cheney and Rumsfeld were doing, who didn't care about lying to Americans and others, but wanted to get public support for their agenda at any cost.
These people look the same from the outside only if you don't care about nuances and/or are anti-American by default and see all their leaders as war criminals anyway.
Shame about Baldwin, it would have been much funnier if he did that in the middle of Fifth Avenue in NYC in a Trump wig.