Don't forget the food service connection in Detroit. Many people in low paying jobs (invisible people) have access to the tarmack. There are places on planes where people don't usually look during cleaning. (Ever checked your life preserver under the seat?) The schedules of the planes are well known and airliners have loop routes that make it simple to plan ahead and leave things on the plane before it gets to Logan airport. The hijackers even knew what seats were theirs. Where were these airliners the last few airports before they became missiles?
It doesn't matter about the security. The guys can obard the plane, and get along just nicely. The cleaning crew can slip the stuff in, and hide it in any number of places.
And it only takes one or two of those. How do you check every airplane before every flight?
That's not how I read the article. Implication is hijackers aborted plan and discarded their tools of evil.
Or hiding them mid-flight to evad a possible search when it became evident that the planes were going to land at the nearewst field.