Your whole assumption is based on the fact that they want to find terrorists when I would put forward that their mission is actually to grow the institutional power. Budget share, it's what drives DC.
I'm sure there's a bunch of non-falsifiable fuzzy arguments you could substitute for this one. How about, "you assume they're looking for terrorists, but really they just don't like the texture of pressure-braised chicken."
I'm really having no problem seeing the situation as portrayed unfolding in a large bureaucracy.
1) Some dude adds pressure cooker to the dumb keywords list that's never found a valid lead, right after the only incidence in history of a pressure cooker being used as an implement of irregular warfare. (Reactive, CYA, ineffective, typical)
2) Whether they send out FBI trucks to people who came up on the filters: I agree that it's stupid. But that doesn't mean they wouldn't do it. How does "Investigated 46,000 terror leads last year" sound on the executive report?
It's not a matter of 'how could they be so smart and so dumb at the same time'. It's different people being smart and dumb.
Eh maybe so. But I've was raised in DC and worked for government many times and my family is in the intelligence industry so it's my own conclusion. I bet many in DC would agree with me.